The invention discloses a low-pressure precision casting method for a magnesium alloy thin-wall shell part, and belongs to the field of non-ferrous metal part casting methods. According to the method, through wax mold manufacturing, sand hanging and sintering, a mold shell with a high surface quality, high size precision and an internal complex structure is obtained, a magnesium melt is prevented from reacting and oxidizing with the mold shell by internally brushing a coating, and the surface quality is guaranteed; then, a pouring scheme is designed, and key process parameters such as low-pressure casting mold filling time and crystallization pressure are reasonably controlled in combination with a mold shell heat preservation measure, an anti-oxidation measure and a filtering device, so that stable flowing and anti-oxidation of a melt in the rising and mold filling process, in-place mold filling under the pressure condition and feeding in the solidification process are ensured; the internal metallurgical quality of the casting is guaranteed, a complex thin-wall structure is achieved, and meanwhile good surface quality and dimensional accuracy are achieved. By adopting the casting method, not only are the metallurgical quality and the yield of the magnesium alloy casting improved, but also materials are saved, the process yield of the casting is improved, and economic benefits are improved.