A 0.2C-1.5Mn-1.5Si-0.6Cr-0.05Nb (wt%) steel is treated respectively by novel quenching-partitioning-tempering (Q-P-T) process and traditional quenching and tempering (Q&T) process for comparison. X-ray diffraction analysis indicates that Q-P-T steel has about 10% retained austenite, but Q&T steel hardly has one. With the increase of compression strain rate from 7 × 10² to 5 × 10³ s^-1, the flow stress of Q-P-T steel increases, which demonstrates the positive strain rate effect, but does not exist in Q&T steel. The characterization of scanning electron microscopy indicates that a large number of long, straight martensite laths in Q-P-T steel will bend or be destroyed by large compressive strain of 35% at 5 × 10³ s^-1. However, relative small compressive strain of about 5% at 7 × 10² s^-1 almost does not have any effect on the original lath morphology. The characterization of transmission electron microscopy further reveals the origin of the positive strain rate effect and the microstructural evolution during dynamic compressive deformation.