Abstract: The reason why the 20SiMn2MoVA steel with low impact toughness was analyzed by using OM,SEM,TEM and EPMA.The improvement was pointed out.The results show that chemical composition had element spot segregation,non martensite microstructure generated and carbide precipitated which resulted in low impact toughness of 20SiMn2MoVA steel after quenching and low temperature tempering.Increase the quenching temperature and cooling speed properly will benefit to decrease the amount of the non martensite and increase the impact toughness.