The work aims to design a fixed-length cutting control system based on STM32 as the control core. The working principle, hardware design, realization of external interrupt tracking program and HMI design were introduced. In the meantime, aiming at the stationarity of tool holder when it returned at high speed, an S-type algorithm based on Sigmoid function was proposed, and the main parameters of tool holder during returning were substituted into the function for Matlab simulation. The test results showed that, the control system met the working requirements. The simulation results indicated that, the frequency curve was continuous and smooth when the tool holder returned. The control system adopting STM32 instead of PLC is featured by low cost, stable function, high real-time performance and stable operation, and can meet the precision requirements of fixed-length cutting.