Preparation method of polyamide-imide copolymer

The invention relates to a preparation method of a polyamide-imide copolymer, reacting trimellitic anhydride acyl chloride, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether and m-phenylenediamine according to equal anhydride/amine mol ratio at 5-25 DEG C for 3 hours to prepare polyamide imide acid; and adding 0.5-3 wt% of blocking agent to react for 1-2 hours, precipitating in water, filtering, and drying at 220-250 DEG C for imidization for 3 hours to prepare the polyamide-imide copolymer. The polyamide-imide copolymer has the characteristics of high temperature resistance, low disproportionation degree, favorable flowability in machining, favorable alkali resistance and the like; the inherent viscosity is 0.5-1.0dl/g, and the Tg is 265-285 DEG C; and the properties of the injected sample piece are as follows: the tensile strength is 152 MPa, the bending strength is 241 MPa, the notched Izod impact strength is 140 J/m, the surface resistivity is 5.0*10 Omega, the volume resistivity is 2.0*10 Omega.cm, the dielectric constant (60Hz) is 4.20, the dielectric loss tangent (60Hz) is 0.026, and the heat-deformation temperature (1.8 MPa) is 278 DEG C.