The invention relates to the field of mask plate manufacturing, in particular to a metal mask preparing method. A traditional yellow light and etching technology is replaced with a laser pattern processing manner, in other words, an opening area is machined through picosecond lasers, accumulated errors generated through a traditional double-face machining production manner are avoided through a single-face machining manner, and accordingly the machining precision is far higher than the accumulated precision of exposure/developing/etching adopted in the traditional production process. The evaporation angle requirement needed by a metal mask can be met by adjusting the laser incidence angle. Meanwhile, the high equipment investment and factory construction expense are avoided.