Steel Q235B is one of the most widely used structure steels in China. There are as yet no toughness parameters of void growth model(VGM model) and stress modified critical strain model(SMCS model) for such a popular material. Taking 4 types of steel Q235B materials extracted from the hot rolled seamless pipe and the matrix, heat affected zone as well as welded zone of cold welding pipe, eight smooth round coupons and twenty four notched round coupons were prepared. A series of uniaxial tensile tests were carried out, and finite element analyses as well as scanning electron microscopic tests were done too. Toughness parameters of VGM model and SMCS model for steel Q235B had been acquired, the characteristic length of steel Q235B was confirm. Experimental verification was made for the predict effect of ductile fracture of steel Q235B by VGM model and SMCS model. Research result shows that both VGM model and SMCS model can accurately predict the ductile fracture of steel Q235B.