Abstract: A batch of 4Cr5MoSiV1 steel modules in a company cracked during forging. The causes of forging cracking were analyzed by chemical composition anslysis, metallographic examination, hardness test, energy spectrum analysis and so on. The results show that the proximate causes of forging cracking were segregation and network like low melting point copper rich phase distributed along the crystal in microstructure of 4Cr5MoSiV1 steel module.