In single photon ranging, increasing repeating frequency can improve the echo signal-to-noise ratio and result in ambiguous distance. To avoid ambiguous distance, an arithmetic based on modulation ranging and time-correlated photon counting theory is proposed, by which the unambiguous time and multiple of laser emitting periods are calculated, and then the unambiguous distance is obtained. This technology needs less computation and is more effective. Extended pseudo-random sequences with variable proportion of“1”are derived from msequence, and they act as noise model and modulation signal. In simulation, ranging results with three different sequences are compared and it shows the pseudo-random sequences with proportion of“1”as 7/8 are random and noise resistant, of which the ranging result is better than that of all“1”sequence and m-sequence