To address the problem that Lamb wave damage probability imaging requires a baseline signal, a baseline-free damage probability imaging algorithm for Lamb waves based on the same propagation distance is proposed. Firstly, the power spectral density of the signal on each path is solved, then the power spectral density values on the paths with the same propagation distance are grouped, the path corresponding to the maximum value of the power spectral density in each group is regarded as a non-damaging path, and compared with other power spectral density values in the same group to construct a damage index, and finally the damage probability imaging algorithm is combined to identify the damage location. The results show that this baseline-free damage imaging algorithm is capable to accurately localize defects within 6% relative error for localizing multiple types of damage within the inspection area.