The invention provides an automatic locking system for a relative phase between an optical resonant cavity and two light beams, belongs to the field of photoelectric technology, and aims to solve the technical problem of easiness in causing complicated operations since an existing locking system for a relative phase between an optical resonant cavity and two light beams has the defects of high cost, large size, poor oscillation resistance and need of manual relocking after lock loss. A signal input end of a proportional amplifying and integrating circuit module is connected with a signal output end of a signal detector; a signal output end of the proportional amplifying and integrating circuit module is connected with a signal input end of an AD conversion module; the signal detector is connected with a system to be locked; a signal input end of the control module is connected with a signal output end of the AD conversion module; a signal output end of the control module is connected with a signal input end of a DA conversion module; a signal output end of the DA conversion module is connected with a signal input end of a high-voltage amplifier; and a signal output end of the high-voltage amplifier is connected with a signal input end of the system to be locked.