Using the double slit diffraction and interference principle, small fluctuation of wavelength leads to refractive index change and generates a obvious phase difference between two diffraction slits. Thus zero level of the diffraction stripe deviates from the optical axis. Small fluctuation of wavelength is monitored in realtime through the change of offset. Experimental results show that 2 pm change of the laser wavelengh can cause displacement of diffraction stripe for about 2.6 μm. Because the displacement of zero level diffraction stripe is detected, effects of light intensity change are avoided which greatly improves the wavelength monitoring resolution.