Polyvinyl chloride (PVC) is widely used as a building material, but it has poor flame retardancy and is prone to combustion. The study doped an appropriate amount of flame retardant magnesium hydroxide (MH) in PVC and compared the effects of granular (MHP) and fibrous (MHW) on the mechanical properties, thermal stability, and flame retardancy of PVC. The results show that under the same doping amount, the comprehensive performance of PVC/MHP is superior to that of PVC/MHW sample group. Compared to PVC materials, the elastic modulus of PVC/40%MHP is increased by 69.8% to 2 080 MPa. The bending strength of PVC/30%MHP reaches the maximum value of 67 MPa, which is increased by 26.4%. The total heat release of PVC/10%MHP sample is decreased by 72.1% to 11.6 MJ/m², and the total smoke production is decreased by 92.9% to 0.06 m²/kg. At 600 ℃, the mass residual rate of PVC/10%MHP sample is 28.6%, and the thermal stability is improved.