Thermoplastic tapioca starch(TPS) was prepared with the mixed plasticizer, formamide and urea (mass ratio 2:1). The introduction of magnesium hydroxide(MH) in TPS could rapidly increase processing torque on a high level(40~60Nm) at 130°C. Studies in the dependence of mechanical properties of thermoplastic tapioca starch/magnesium hydroxide composites(MHTPS) on the MH content from 0 to 20phr, the initial tensile strength and elastic modulus were increased up to the maximum 3247 MPa and 48.5 MPa at 20phr MH content respectively, while the elongation at break was reduced from 72% to 2.5%. TG mass loss curves showed that thermal stability of this composites had great improved under 500°C. The limited oxygen index(LOI) of MHTPS reached to 31% while the content of Mg(OH)2 was 20phr, and then the result of vertical flame testing achieved UL94 V-0 level.