The invention discloses a hydroelectricity generator group primary frequency modulation theoretical motion electricity quantity calculating system and method. The method comprises the following steps: an object guide vane opening degree signal of a water turbine group can be obtained via an electronic regulator model according to frequency deviation and active power deviation or guide vane opening degree deviation, an actual guide vane opening degree signal can be obtained via a guide vane execution mechanism model according to the object guide vane opening degree signal, theoretical active power can be obtained via a water turbine prime mover model according to the actual guide vane opening degree signal, and the water turbine prime mover model is an ideal water turbine model by which only rigid water hammer is considered; a hydroelectricity generator group primary frequency modulation theoretical motion integral electricity quantity can be obtained via a theoretical motion integral electricity quantity calculating module according to a relation expression between theoretical active power, time corresponding to the theoretical active power and a theoretical motion integral electricity quantity. According to the hydroelectricity generator group primary frequency modulation theoretical motion electricity quantity calculating system and method, the theoretical active power of the hydroelectricity generator group can be obtained via a simulation model of the hydroelectricity generator group, the theoretical motion integral electricity quantity can be obtained via a relation between the theoretical active power and the time, and accuracy of the theoretical motion integral electricity quantity which is obtained via the mode can be improved.