The invention relates to a deflecting flow surface parameter design method for tidal flow reversal of a steep slope seawall. The method comprises the following steps of (1) designing an overall layout of a deflecting flow surface of the seawall; (2) designing a diversion section; (3) designing an arc section; and (4) designing parameters of a deflecting flow section: analyzing and determining the length Lt of the deflecting flow section, a deflecting flow angle theta and the position of a starting point of the deflecting flow section. The parameters of the deflecting flow section can be analyzed and preliminarily selected by applying a simplified formula firstly and then are determined by applying SPH numerical simulation. The simplified calculation formula is defined in the specification. The method is a deflecting flow surface parameter design method based on a combination of the arc section and the deflecting flow section of the steep slope seawall; and basic theoretical support and calculation methods are provided for effectively solving the difficult problems of tidal watching safety and tidal bore landscape maintenance of the steep slope seawall.