Objective Removable expanders have been a typical treatment option for children with transverse maxillary deficiency in the mixed dentition period，but they may affect dental root development and cause root resorption. We aimed to use cone-beam computed tomography（CBCT） and Exam Vision to preliminarily assess the effects on the root development and root resorption of permanent teeth caused by maxillary expansion with removable appliances in the mixed dentition period. Methods We enrolled 17 patients treated with removable maxillary expansion during mixed dentition（experimental group；mean age，10 years） and 20 patients treated with functional correction without maxillary expansion（control group；mean age：10 years） in the Department of Orthodontics of Stomatological Hospital of Chongqing Medical University. CBCT was taken for all the subjects before and after treatment. We used Exam Vision to measure the length of the root and the width of the apical foramen of the maxillary central incisors and maxillary first molars before and after treatment. SPSS 26.0 was used for statistical comparisons. Results The width of the apical foramen：The root of right maxillary central incisor was （0.52±0.35） mm in experimental group and （0.98±0.78） mm in control group；The root of left maxillary central incisor was （0.47±0.39） mm in experimental group and （1.09±0.93） mm in control group；The palate root of the right maxillary first molar was （0.43±0.28） mm in experimental group and （0.61±0.34） mm in control group；The palate root of the left maxillary first molar was （0.59±0.24） mm in experimental group and （0.59±0.40） mm in control group；The degree of apical foramen closure was lower in experimental group than in control group. The length of the root： The root of right maxillary central incisor was （0.48±0.77） mm in experimental group and （0.83±1.35） mm in control group；The root of left maxillary central incisor was （0.31±0.84） mm in experimental group and （0.78±1.16） mm in control group；The mesial root of the right maxillary first molar was （0.37±0.72） mm in experimental group and （0.42±0.79）mm in control group；The distal root of right maxillary first molar was （0.10±0.92） mm in experimental group and （0.70±1.09） mm in control group；The palate root of the right maxillary first molar was （0.43±0.96） mm in experimental group and （0.70±1.12） mm in control group；The control group showed greater increases in the length of the root of the maxillary central incisors on both sides and the maxillary first molar on the right side. The mesial root of the left maxillary first molar was （0.59±0.79） mm in experimental group and （0.28±0.68） mm in control group. The distal root of the left maxillary first molar was （0.43±0.70） mm in experimental group and （0.40±1.06） mm in control group；The palate root of left maxillary first molar was （0.62±0.84） mm in experimental group and （0.61±0.97） mm in control group；The experimental group had an insignificantly greater increase in the length of the root of the maxillary first molar on the left side. The length of root divided by the closure status of apical foramina：The palate root of the right maxillary first molar was （0.40±0.97） mm in the closed group and （-0.06±0.72） mm in the unclosed group. The increasing amount of root length in the closed group was greater than in the unclosed group. The palate root of the left maxillary first molar was （0.20±0.59） mm in the closed group and （0.78±0.91） mm in the unclosed group. The increasing amount of root length in the unclosed group was greater than in the closed group. Conclusion For treating transverse maxillary deficiency during mixed dentition，removable expansion has no significant effects on the root development and root resorption of permanent teeth，suggesting that orthodontists can use removable appliances for treating children with transverse maxillary deficiency as appropriate. The optimal age for maxillary expansion should be further studied.