This paper introduces the design of front circuit in cleaning grain loss monitoring and control system for a new electric combine harvester. This front circuit mainly includes signal conditioning circuits which are amplification circuit, anti-jamming circuit and filter circuit for transforming sensor output signal. 8031 single-chip is used as its core and implemented automatic monitoring of the cleaning grain loss. Owing to adopt the modular design method and mature technology, the circuit is powerful in function and easy to realize and has extensive value in use. The simulation results show that the effect is excellent and performance is stable.