The fluctuation of laser brings many troubles when tunable diode laser absorption spectroscopy (TDLAS) is used in the real-time measurement of gases. For eliminating this influence, a novel method, in which a digital proportion integration differentiation (PID) algorithm and a software lock-in amplifier designed by using Lab-VIEW are employed, is introduced to lock the laser to the center of the absorption line. In order to test the system, three intimate absorption lines of methane near 1.653 μm are chosen by using fiber-coupled distributed feedback (DFB) diode laser. A 24-hour continuous measurement is implemented. Relative results are presented and discussed. The detection limit is lower than 1.8×10-8 while the second harmonic signal measurement is utilized. This method can be applied to the detection of trace gases by using other long path length cells (CEAS) or combustion diagnosis.