A real-time measurement of gases based on QEPAS is introduced. For eliminating fluctuation of laser, a simple method, in which a digital PID algorithm and a software lock-in amplifier designed by using Lab-VIEW are employed, is utilized to lock the laser to the center of the absorption line. In order to test the system, an absorption line of water vapor near 1.396 m is chosen by using a fiber-coupled distributed feedback (DFB) diode laser. An over-40-hours continuous measurement is implemented, and the relevant results are presented and discussed. The detection limit of the system is 1 ppm. This method can also be used to measure the trace gases and monitor the industry gases.