The invention relates to the technical field of cable-stayed bridges and in particular relates to a method of controlling rigidity of a long span cable-stayed bridge of a railway through a stay cable. The method analyzes influence rules of the cable-stayed bridge on bridge dynamic characteristic, vehicle dynamic response and bridge dynamic response under different numerical values by establishing a long span cable-stayed bridge model of the railway and a bridge model by means of a dynamic analytical method, and performs judgment by taking an evaluating index system of a vehicle-bridge system as an index to obtain influence conditions on the integral rigidity of the bridge under different rigidity numerical values so as to further obtain a reasonable value range of the rigidity of the stay cable, thereby providing basis for valuing and designing the rigidity of the stay cable; the integral rigidity of the long span cable-stayed bridge of the railway is better controlled through the rigidity of the stay cable, so that the long span cable-stayed bridge of the railway achieves a breakthrough on the rigidity control technology; the technical problem is solved, so that the long span cable-stayed bridge of the railway is promoted, and a lot of engineering investment cost is saved.