The invention relates to a runoff data free region confluence calculation method. The method comprises the steps of collecting finished river basins which have history rainfall runoff data, wherein the unit line confluence prediction effects of the river basins are relatively good, and storing confluence schemes and river basin geographic information in advance; when flood confluence calculation of runoff data free river basins is carried out, establishing number-time distribution curves for the net rainfall drop number of grids in data river basins and data-free river basins, carrying out generalization by employing triangles, carrying out similarity calculation on generalized triangle processes, thereby searching similar river basin sets, carrying out time-space conversion on unit line processes of the similar river basin sets, and synthesizing converted unit line sets according to the most unfavorable principle with high peaks and short time, thereby obtaining the unit lines of the runoff data free river basins. According to the method, on the basis of accumulating a great deal of river basin prediction, similar river basin selection, and time-space conversion and synthesis of the unit lines are carried out by employing targeted criteria, artificial subjective judgment errors can be effectively avoided, the efficiency is improved, and the foundation is laid for prediction of the wide-range data free regions.