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The composition structure ,the work principle and the soft design of control system for the heating furnace are introduced briefly. In the system,the frequency signal is gotten from V/F converter used as an A/D (Analog and Digital) converter, and then it is sent to high-speed counter of PLC to measure the temperature of heating furnace. The PWM (Pulse Width Modulation) curve produced by PLC are used to control the temperature of heating furnace as a D/A(Digital and Analog) converter.In the software design, the PID algorithm is applied to form a closed-loop control. Furthermore, the piecewise fitting method is used to make up for the defects of conventional PID algorithm by modifying the curve of frequency and temperature .Experimental results show that these method reduce the cost of control system via replacing expensive analog A/D module and D/A module ,and also obtain better steady-state accuracy and dynamic characteristics．