The second harmonic is used when using wavelength modulation technology to design gas sensor based on optical spectrum. Whereas phase difference between gas sensing signal and double frequency reference signal has great influence on the measured result of second harmonic. A reference optic fiber is used to synchronize the phases. There are two parts in this technology: phase shift circuit and reference optic fiber. Phase shift circuit consists of digital circuit which is used to compensate circuit delay and is easy to modulate. Reference optic fiber which does not pass by gas cell is added in the optical path. It makes sure that the phases between measured signals and referenced signals are synchronized no matter how the length of the optic fibers changes. In the design of phase synchronization using double optical path based on wavelength modulation,the error of meausred result keeps lower than ±10% with phase of measured signals changing from 0° to 90°. It not only simplifies the calibration of the sensor, but also makes possible to install the sensor at any distance site.