There are two different frequencies corresponding to the same distance caused by the randomness of observation time, based on the analysis on the different frequency of sawtooth frequency modulated continuous wave ranging system. By means of analysis on the difference frequency signal in time domain and frequency domain, it is found that this kind of ranging ambiguity can be settled by mixing frequency twice. The output frequency of the signal processing unit corresponds with the distance one to one at any observational time. The effectiveness of this signal processing method is verified by the signal spectrum and time-frequency curves emulation using the simulation software.