Welding heat simulation technology is used to simulate the effects of different heat inputs of manual arc welding on the microstructure and performance of the coarse grain region of steel for railway freight cars in harsh environments. The test results show that the microstructures of coarse grained regions with different heat inputs are proeutectoid ferrite growing into the crystal along the g rain boundary,there are granular bainite,acicular ferrite and a little lath martensite in the crystal. The lower heat input,the more lath martensite and the finer the grain. The hardness of the test piece reduces as the heat input increases. All coarse grained regions of the thermal simulation test pieces with different welding heat inputs have good impact performance,especially the low temperature impact performance is excellent. Fine grain strengthening is a main reason that steel for railway freight cars in harsh environment has high strength and good toughness.