The invention provides a novel method used for phase lock voltage subtraction of micro water vapor detection, and belongs to the technical field of optical fiber sensing detection. Wavelength modulation is conducted on a laser, and signals passing through gas-absorption peaks and non-absorption positions are detected through the phase locking technology and are processed. An achieving system comprises a temperature controlling and driving module of the laser, the laser, a one-to-one optical fiber coupler, a gas chamber, a photoelectricity detector, a current and voltage conversion module, an automatic gain control amplifier, a subtracter, a phase locking module and a signal processing and detecting module. According to the novel method, as the phase locking module is used and detection signal subtraction is conducted before phase locking, the signal to noise ratio can be greatly improved, and gas concentration signals are effectively extracted. Automatic gain amplification can overcome the detect that vibration of optical fibers, power fluctuation of the laser and other factors cause light intensity signal changes, and stability of the system can be improved. In addition, the system has the advantages of being small in size, simple in structure, low in cost and the like.