We present the locking laser frequency on a Doppler absorption peak for the small size coherent population trapping (CPT) atomic frequency standard by using digital control circuit with a field-programmable gate array (FPGA) core. With the similar scheme to lock microwave frequency, it is realized of the close loop locking of the CPT frequency standard. There are many advantages of the digital servo system based on FPGA using Verilog language, such as compact structure, flexible parameter adjustment, easy debug, low power consumption, and small temperature coefficient, etc. Benefited from the characteristics of programming and digital circuit, the system is good in transplantation and consistency. With this digital servo scheme, the realized CPT frequency standard achieves 4-W power consumption and 6×10－11τ－1/2 frequency stability, which proves to be a feasible scheme.