Based on the fast data processing ability of field programmable gate array (FPGA), a novel digital servo control system overcoming the paradox between the locking precision and the control range with double proportional integral derivative (PID) algorithm is introduced. The digital servo system improves the persistent time of phase locking greatly. Using sinusoidal and integrating scan for the length of optical cavity respectively, the auto-search function is realized when the system is out of locking. The search time is shorter with the former method than the latter; while the latter method is more stable and has longer persistent time. Compared with analog servo system, the locking precision of digital circuit is slightly lower, which is limited by the electronic noise and processing speed of FPGA. However, the digital servo system has obvious advantage in locking persistent time.