The invention relates to a multi-machine differential equation calculation method for a water turbine with a shared pipe, belonging to the technical field of analysis and control of the stability of a water turbine and a hydroturbine. The method comprises the following steps: calculating hydro-mechanical coupling of a shared pipe by virtue of dynamic parameters of a forked pipe, establishing a single-machine single-pipe elastic water attack differential equation model by adopting the forked pipe as a core, and calculating transient changes of a water head and a flow rate of a water turbine at the end of the pipe by employing a multi-machine simultaneous calculation method. According to the method, the hydro-mechanical coupling of the shared pipe is calculated by employing the dynamic parameters of the forked pipe, and the complete hydro-mechanical coupling transient differential equation model is established according to a multi-machine calculation mode, so that the problem of the consistence of the hydro-mechanical coupling in the differential equation iterative calculation can be solved; and the simulation calculation shows that the method has relatively high calculation precision.