Light frequency shift is one of the important factors which influence the performance of the standard frequency of a coherent population trapping (CPT) atomic clock. Usually, it takes a long time to optimize the light frequency shift behavior of a CPT atomic clock. With field programmable gate array, a digital quadrature demodulation method which considerably reduces the time and amount of work to optimize light frequency shift behavior of a CPT atomic clock is developed. Applying the method to the production of the CPT atomic clocks, the debug period and workload can be reduced.