The invention discloses an etching method of a silicon substrate forming an MEMS (Micro Electro Mechanical System) device, which comprises the following steps: 1. providing the silicon substrate, and respectively depositing a first hard mask layer and a second hard mask layer on the upper surface and lower surface of the silicon substrate; 2. forming a patterned photoresist layer on the first hard mask layer through a photoetching process; 3. taking the patterned photoresist layer as a mask, and etching the first hard mask layer through a dry etching process to form a patterned first hard mask layer; 4. eliminating the patterned photoresist layer; 5. depositing an SiN layer on the second hard mask layer; and 6. taking the patterned first hard mask layer as a mask, and etching the silicon substrate through an anisotropic wet etching process.