Abstract: The 400 MPa grade ultra-fine grained steel is characterized by its higher cleanness and ultra-fine grain. The 400 MPa grade ultra-fine grained steel were welded by tungsten argon shielded arc welding,the microstructure and mechanical properties of welded joints under different cooling conditions of air cooling and water cooling were analyzed. The results show that the cases about grain size increasing greatly,strength decreasing and softening area appears do exist in HAZ of welded joints of 400 MPa grade ultrafine grained steel. Using spraying water to speed up the cooling rate in the welding process,could suppress the cases about grain size increasing in HAZ of welded joints effectively,improve the microstructure compositions,and enhance the contents of bainite and low-carbon martensite,then improve the comprehensive mechanics performance of the welded joint,so it could satisfy the actual operation requirements.