The second harmonic generation(SHG) technique can effectively extend the existing laser wavelengths range. It is widely applied in atomic, molecular physics and optical physics, biomedicine and other fields. Long-term stable locking of the optical frequency doubling cavity is a significant technology for the continuous wave frequency doubling technique, which is directly related to the laser output power stability and reliability. At present, the commercial SHG lasers have off-locking phenomena at different level. In this paper, an automatic locking scheme based on analog integral scanning technique was proposed. With this technique, the system could continuously work in the absence of artificial regulation, and no additional power monitoring and scanning signals were required. The system could automatically restore the locking state after it is out of locking. As a result, the long term and stable SHG laser output is obtained.