To recover the missing frequency information in optical-frequency-comb-based down-converted signal receiving, this paper introduces a frequency recovering method based on frequency-phase-mapping. An optical time delay line is used to generate a certain time delay and it builds a frequency-phase-mapping between signal’s frequency and down-converted component’s phase, which can be acquired by data processing. Thus, the frequency can be calculated through the measurable phase and certain time delay. Then, this paper analyzes parameters such as time delay on frequency recovery, estimates the limit requirements of this method for the uncertainty of phase measurement, and finally gives the setting strategy of key parameters in the specific implementation of this method. The number of signals under test is theoretically unlimited as long as the down-converted components are not aliased. Compared to related works, this method has better performance in multi-signals recovery, because its expense in time consumption and data processing is nearly constant as the number of signals increases.