Nowadays, in the industrial production, the cascade control is an important issue. With the development of science and technology, the stability and dynamic process requirements of the industrial field to the cascade control are also higher and higher. A scheme of cascade control system based on A3000 process control experimental system was proposed, which uses PLC as the controller. Liquid level signal was sent to PLC by liquid level transmitter, using PID algorithm to process the data and then output to the electric control valve to achieve the liquid level control. Taking the Kingview as upper unit configuration software, the experiment constructed a configuration screen for realtime monitoring of dualtank. The scheme can achieve high accuracy of control and realtime status. The adjustment time and steadystate error meet the control requirements.