In conventional DOA methods, the subspace fitting algorithms have been widely used for its high resolution and high estimation accuracy. However, when subspace algorithm works, it needs to predict the exact number of sources, otherwise the performance of the algorithm will be seriously affected. In the view of the problem of high precision direction-finding under conditions where the number of signals is unknown, a DOA algorithm based on spatial secondary projection is proposed. Firstly, this algorithm constructs the primary-projection spatial spectrum by the relationship between vector and its spatial projection. Secondly, it takes a secondary-projection to removes the possible pseudo-peaks by taking the advantage of the uniqueness of the vector representation by complete spatial vector basis. The computer simulation results show that the spatial secondary projection algorithm can realize the accurate measurement of the coherent signals’ DOA without predicting the number of signal sources, and this algorithm has high spatial resolution and algorithm stability under the conditions of small aperture and limited number of snapshots.