A P-type high-voltage MOS transistor with built-in protection is composed of P-type substrate, N-type epitaxial contact holes, source, drain, field oxide layer, polycrystal grid, grid oxide layer under said polycrystal grid, the oxide layer on said source, drain field oxide layer and polycrystal grid, P-type drift area, leading-out aluminium wires, N-type impurity area between substrate and N-type epitaxial contact holes (and source and said drift area), and the N-type protecting trap under the field oxide layer for higher breakdown voltage. |