Abstract: In this paper, a novel MEMS multi-mask process for precision bulk silicon process with big steps and big aspect trench was proposed.After repeated thin photoresist（PR） photolithograph on substrate, multi-mask layer included SiO₂/ Si₃N₄/PR with different graph were created.Each layer can be used for substrate etching and can be removed after etching.This process solved the problem of PR coating and photolithograph on substrate with deep pits and trenches in MEMS device fabrication.Combined with Deep Reactive Ion Etching（DRIE）, wet etching and other process, it can be applied in some MEMS process such as multi-step, small structure in deep pits, micro-strcuture releasing.