The feedback level C in the optical feedback interferome try system indicates the strength of the light fed back to the laser cavity.It plays a particularly important role in determining the running state of the laser diode and displacement measurement of the object.A measurement algorithm based on the opt ical feedback phase spectrum characteristics is presented in this paper.Based on phase unwrapping operations ,the phase information is acquired from the laser self-mixing interference (SMI) signal.Then the phase informatio n is converted to the frequency domain to analyze the spectrum characteristics.The feedback level C can be ac quired from the spectrum characteristics.And the simulation results verify the algorithm′s validity an d accuracy.In order to verify the method′s precision in practice,the self-mixing interference measurement system of feedback level C is constructed.In this measurement system,the relative standard deviation (RSD) of the measurement can be only 2.11%.The experimental results also show that the algorith m can be used in all the three kinds of feedback mechanisms.The measurement rand om error is small,and the method is simple and convenient for operation.