The invention discloses a thin-walled heat resistant steel exhaust gas manifold casting method, and belongs to the technical field of heat resistant steel exhaust gas manifold casting. The casting method comprises the following steps of core manufacturing, modeling, smelting, pouring and sand shakeout. Through the reasonable design of the amount of molten iron entering a pouring channel and the section ratio of each unit in a pouring system and strict control of the iron mold sand covering layer thickness, the mold cavity exhaust gas system and pouring temperature and the sand shankout box opening time, the thickness of the prepared exhaust gas manifold is 3.2-3.6mm, the wall is thin, the grain size level is 5.5, and the technical property reaches the required index. The prepared exhaust gas manifold has the thin wall and low weight, is stable in property, solves the problem that a common wet sand technology is difficult to form, effectively lowers the integral weight, and achieves light-weight design of an engine.