The mechanical properties of the boron steel used for automobile actually depend on the phase ratio of the microstructure, and the phase ratio is determined by soaking temperature and time. To obtain the expected phase ratio, it is needed to study the relationship between the experimental parameters and the phase ratio. The phase ratio of the material with different soaking temperature is studied with the cooling rate of 30 ℃?s-1, and the key parameters and method of controlling phase ratio combined with the empirical formula are given. It could provide theoretical reference to obtain the microstructure of provisions phase ratio in practical engineering.