The evolution of microsturctures and influence factors of ultrathin hot strips of low carbon steel produced by compact strip production (CSP) techniques were investigated. The steel blocking samples of CSP six-passes were obtained, and microstructures at the different positions of workpiece for each pass were studied. At the same time, an explicit finite element technique was used to reveal the continuous rolling process. By combining experiment results with simulation analysis, the effects of plastic strain, temperature, precipitation and interval time on evolution and refinement of crystal grains have been investigated. The results are useful for the development of high strength hot strips.