The machining precisions of phase mask directly affect the performance of a stellar coronagraph imaging system. The error formula of six-level phase mask is derived. The influences on the extinction and the achromatic performance are analyzed when small errors of phase mask are induced. The numerical simulation results show that the errors of phase horizontal hight of phase mask and tilt angle variation can reduce the extinction performance for the monochromic light of specific wavelength. However, in comparison, the coronagraph extinction performance is more sensitive to the tilt angle errors. With in the scope of the broadband, considering the achromatism ability, the level height error can only affect its central wavelength, whilethe small errors of tilt angle can greatly degrade its achromatic performance.