General description, mathematical analysis and simulation research results of an unambiguous radial velocity measurement method concerning the space surveillance low pulse repetition frequency (PRF) radars are presented in the paper. The low PRF radars, used primarily for searching and precisely target tracking, obtain target’s accurate range as the primary measured parameter and are ambiguous in radial velocity measurement. In this paper, unambiguous target’s radial velocity values can be obtained in low PRF radars through the digital intermediate frequency locked loop (DIFLL) and spectrum peak search (SPS). The simulation results verify the availability and high practicable value of the method.