Abstract: Fierce sea clutter submerges the slow weak target echo which makes the target detection difficult. To address the problem, a Constant False Alarm Rate (CFAR) detection method based on orthogonal projection is proposed in this paper. Utilizing the correlation of the clutter between neighbored range cells, this method suppresses clutter by orthogonal projection at first, and then carries out the CFAR detection. The structure chart of the CFAR detector using orthogonal projection is given. And the performance of clutter suppression and computation amount for orthogonal projection method and singular value decomposition method are compared. Theoretical analysis and simulation results with the experimental data show the superiority of the proposed method.