In order to realize high-precision testing of aspherical surface, the key technology for fabricating computer generated hologram (CGH) by laser-direct writing (DWL) is studied in this paper. To obtain the minimum line-width deviation, the influences of defocus and the wet-etching processes on the line-width are firstly investigated. The position errors under different line-widths are obtained by analyzing the relationship of line-width error and the position error. According to the above-obtained results, a CGH with diameter of 80 mm and the minimum line-width of 1.8 mm is successfully fabricated. Testing results show that the wavefront error is only 0.011l, achieving the l/100 level. The fabricated CGH is expected to be used in the high-precision testing of asphercal surfaces.