The new generation TMCP process based on ultra fast cooling has recently developed rapidly. In order to develop the low yield ratio constructional steel, the effects of cooling finishing temperature on microstructure and yield ratio of steels were studied primarily. The results show that the high strength and low yield ratio can be achieved by multiphase including M-A hard second phase and bainite ferrite soft matrix. When UFC final temperature is 521°C, the uniform M-A phases with volume fraction of 22.5% are distributed on bainite ferrite matrix, and then the yield strength is 570MPa, tensile strength 760MPa, yield ratio 0.75, and percentage elongation 22% with the Charpy impact energy 284J at -40°C. All these indexes come up to the relevant standards. The steel with lower yield ratio can be subjected to larger plastic deformation before the necking instability.