Abstract: Signals reflected by structural damage are inevitably coherent during wave propagation in Lamb wave based structural damage detection, which will decrease detection accuracy. Considering the superiority of spatial spectrum estimation in signal source identification and location, especially in the signal direction of arrival(DOA), spatial spectrum estimation was applied to locate structural damage so as to obtain accurate damage information in Lamb based structural damage detection. Thus, a coherent signal subspace method(CSM) in spatial spectrum estimation was developed so as to solve coherent signal problems. The numerical simulation of a damaged aluminum plate illustrated the feasibility of the CSM in the spatial spectrum estimation of coherent signals. It indicates that no matter the coherent signals were reflected by the boundary and structural damage or the two types of damage, all the structural damage information could be well obtained by this method.