Abstract: 4Cr5MoSiV1Sr1W1 die steel was subjected to Ce modification. The effects of different amount of Ce on the as-cast microstructure and mechanical properties of 4Cr5MoSiV1Sr1W1 die steel were investigated by microstructure observ ation, hardness test and tensile test. The results show that the as-cast microstructure of 4Cr5MoSiV1Sr1W1 steel without modification is coarse, and the carbides and inclusions at the grain boundaries are aggregated in network, and the microstructure segregation is serious. Ce element has a good metamorphic effect on 4Cr5MoSiV1Sr1W1 steel. The test steel with 0.04wt% Ce has the best microstructure refinement effect and the segregation improvement is the most obvious. With the increase of Ce element, the hardness and tensile strength of the test steel first increase and then decrease, and the elongation first decreases and then increases and then decreases. The hardness and tensile strength of 4Cr5MoSiV1Sr1W1 steel with 0.04wt% Ce are highest, and the plasticity is also at a high level, and the comprehensive mechanical properties are best.