Aiming at the problems of multi-path sources in transmition and the active interference which might cause the coexistence of uncorrelated sources and coherent sources, an ordinal DOA estimation method is proposed under non-stationary noise field. The uncorrelated sources are firstly estimated by using the conventional spatial spectrum estimation method, then the noise is eliminated by the generalized differencing method and the uncorrelated sources are displaced by exploiting the property of oblique projection, so the only remained coherent sources can be renewed to full rank by the technique of the modified spatial smoothing algorithm. Compared with the traditional method, the number of sources resolved by this approach can exceed that of the array elements avoiding repeated estimation of the correlated sources. Simulation results demonstrate the effectiveness and correctness of the proposed method.