Objective To automatically segment four body components at the T4 thoracic veertebra plane on chest CT in the newly diagnosed multiple myeloma （MM） patients by deep learning model， and to discuss the correlation between the four body components and the prognosis of the MM patients. Methods The retrospective analysis was conducted on the clinical data of the MM patients diagnosed in our hospital from January 2017 to December 2021. The clinical informations such as age， gender， weight， height， and body mass index （BMI） of the patients were collected. The laboratory data of the patients were collected， including serum levels of lactate dehydrogenase （LDH）， calcium （Ca）， creatinine （Scr）， albumin （Alb）， hemoglobin （Hb）， β2-microglobulin （β2-MG）， and serum free light chains. The chest CT images of 79 regularly evaluated MM patients detected by deep learning model were divide into four body components： pectoralis major， pectoralis minor， subcutaneous fat， and mediastinal fat. Image J software was used to detect the areas of the four body components at the T4 thoracic vertebra plane， and their correlation with the prognosis of the MM patients was analyzed by Kaplan-Meier survival analysis. Results The univariate analysis results showed that the area of subcutaneous fat， serum Ca levels， Scr levels， and International Staging System （ISS） stage were related to the overall survival （OS） of the MM patients （HR=2.260， 95% CI： 1.116-4.578， P=0.024； HR=2.088， 95% CI： 1.007-4.327， P=0.048； HR=2.209， 95% CI： 1.105-4.414， P=0.025； HR=1.730， 95% CI： 1.040-2.879， P=0.035）. The multivariate analysis results showed that the area of subcutaneous fat among the four body components was an independent risk factor affecting the prognosis of the MM patients （95% CI： 1.228-5.782， P=0.013）. The Log-Rank test results showed that compared with high subcutaneous fat area group， the OS of the patients in low subcutaneous fat area group was decreased（P=0.018）. There was no significant difference in OS of the patients with different genders between high subcutaneous fat area group and low subcutaneous fat area group （P>0.05）. In the patients without hematopoietic stem cell transplantation， compared with high subcutaneous fat area group， the OS of the patients in low subcutaneous fat area group was decreased （P=0.037）. Conclusion Among the four body components at the T4 thoracic vertebra plane， the area of subcutaneous fat is related to the OS of the MM patients and it is an independent risk factor for the prognosis of the MM patients， while the areas of mediastinal fat， pectoralis major， and pectoralis minor have no predictive value for the prognosis of the MM patients.