In view of the problem of target echoes interfered by strong direct path-wave in bistatic sonar, a high resolution algorithm of direct path interference suppression is proposed when the deployment is known. The method divides the space of N dimension into two orthogonal subspaces under the condition of constraint, the weight matrix of smoothing MVDR (Minimum Variance Distortional Response) algorithm is decomposed into these subspaces, and the optimal weight is obtained by output power minimization in the constraint subspace. The simulation results show that a deeper null appear at the constraint direction compared to conventional beam when the probable direction just is knew, and it yields distortionless response of multi-coherent unknown signals. The algorithm not only suppresses the direct path interference effectively, but also possesses the capability of high resolution for multi-coherent unknown signals.