The invention provides a digital wave length coding optical absolute displacement sensor. An incidence light spot enters a coding grating ruler to obtain a wave band coding signal, the wave band coding signal is demodulated to be converted into a digital electrical signal, and the one-to-one relationship between the displacement and wave length codes is built. A demodulating system directly gives out a digital signal, microminiaturization processing of the demodulating system is facilitated, a sensor part and a signal transmission part are both in optical signal transmission, and anti-electromagnetic interference can be achieved. Whether the signal exists or not is judged according to different kinds of wave length information and does not depend on the strength information of the optical signal, the problem of error codes caused by signal intensity changes generated by vibration, optical fiber transmission losses and light source intensity fluctuation is solved, compared with a wave length coding sensor adopting a grating distance variable optical grating at present, the digital wave length coding optical absolute displacement sensor has the advantages that a continuous light source is not required, a color sensing element with large light sensing area is used at a demodulating part for replacing a linear array CCD and the like, the response frequency is improved, and meanwhile the high-and-low temperature adaptability of the sensor is improved.