The invention provides a complex water area grid regeneration method based on boundary sewing and expansion, which comprises the following steps of step 1, extracting the boundaries of the existing grids, analyzing the existing grids, judging which point is a boundary node, and then extracting the boundaries of the existing grids required to be sewed or expanded; step 2, carrying out the boundarysewing or boundary expansion, when the distance between the boundaries of the two existing grids needing to be connected is relatively shorter, for example, the distance S is smaller than 1.5 D, directly sewing the boundary points of the two grids through triangles or quadrangles; when the boundaries of the two existing grids needing to be connected are farer, for example, the distance S is largerthan 1.5 D, or only one existing grid needs to be expanded, adding the connecting lines, and after a closed area is formed, expanding the grids; and step 3, carrying out the grid regeneration, aimingat a new triangular grid generated by sewing the existing grids and the boundaries or expanding the boundaries, numbering again according to the unstructured grid codes, so that the integral unstructured grid can be obtained, the existing grids can be fully utilized, and the workload of the grid regeneration is greatly reduced.