In order to facilitate the operability of continuous variable quantum optical experiment, an automatic locking system with microcontroller (MCU) is designed and is used in the cavity length locking of the optical mode cleaner. We use analog to digital conversion to identify the signal, the MCU to complete the relevant data analysis and calculation, and then digital to analog conversion to complete the signal output. Finally, the automatic locking of cavity length is realized. When the cavity is unlocked, the system can automatically switch working mode to achieve re-locking. The re-locking time is less than 01 s, and the fluctuation of intensity is less than 1％.