A maximum likelihood algorithm is proposed to locate the scan-based emitter in the condition of unknown scan rate. This algorithm projects all time difference measurements to the target state space, and then maximum likelihood estimates of the scan rate and the target states can be obtained by modification, iterative calculation and fusion, avoiding the poor accuracy of jointly estimation for moving target. Furthermore, the instantaneous localization algorithm is modified by deducing and analyzing error items for the characteristics of scan-based emitter. Simulation results show that the proposed algorithm and its modification are effective, and the latter localization accuracy approaches Cramér-Rao lower bound.