Abstract: The effect of pre-quenching treatment on microstructure evolution and mechanical properties of ultra-high strength QP steel after quenching and partitioning（QP） treatment was studied by means of quantitative metallography, SEM,TEM and XRD techniques. The results show that compared with the sample without pre-quenching, the sample with pre-quenching at 950 ℃ has refined lamellar microstructure, the retained austenite is mainly film-like and it also has a small amount of blocky ones. The microstructure is beneficial to play the TRIP effect and hinder the crack propagation, thus,improving the strength and ductility of the steel, and an ultra-high strength of 1200 MPa is accompanied by an elongation close to 20%. However, when the pre-quenching temperature is 990 ℃, the microstructure of the experimental steel is equiaxed, the martensitic tempering degree is higher, resulting in the domination of blocky retained austenite, which is unfavorable to play the TRIP effect. While the decrease of the amount of bainite and secondary martensite further reduces the strength and ductility of the experimental steel.