The invention provides an installation method for a sternpost and tail fins. The method includes the steps that a horizontal preassembling platform is erected, and a cross coordinate center line and angle lines are drawn on the horizontal preassembling platform; center cross lines of the front end face and the rear end face of the sternpost are drawn; the sternpost is placed on the horizontal preassembling platform with the rear end face of the sternpost as a base plane and is leveled, and the center cross line of the sternpost is right opposite to the cross center line of the horizontal preassembling platform; the tail fins are preassembled on the sternpost, the maximum cross sections of the tail fins and the sternpost are all located on the same plane, the most end points of the maximum cross sections of the tail fins are projected to the horizontal preassembling platform, coordinates of the projection points are compared with theoretical coordinates, and a closing opening between each tail fin and the sternpost is adjusted till the two corresponding coordinates coincide with each other; after preassembling is conducted, rib position preassembling inspection lines coinciding with the maximum cross sections and joining lines at the positions of the closing openings are drawn; and according to the rib position preassembling inspection lines and the joining lines, the sternpost and the tail fins are assembled and welded. By the adoption of the method, the sternpost and the tail fins can be guided to be directly assembled on a field, the installation accuracy is improved, and an installation period is shortened.