The Steam drum is an important hub for the boiler system to be heated and generate steam, and it is an important task for the safe and stable operation of the boiler system to control the liquid level of the boiler drum to be within the set range. In this paper, the dynamic relationship between the steam drum level and the supply water and steam consumption is studied, and the automatic control system of the boiler drum level is designed based on the Siemens S7-300 PLC by using the three-impulse control method, combined with the differential antecedent PID control algorithm. After the system was applied into operation, it effectively improves the control quality of steam drum liquid level, which acquires good application effect.