The invention relates to an isolated network distinguishing method of a hydroelectric generating set. The method is beneficial for effectively distinguishing actual measurement modeling with regard to an isolated network running set, a prime motor for stable calculation of an isolated network set power system and a model and a parameter of an adjustment system thereof are determined, at least two sets of grid-connected control mode parameters are arranged with regard to a speed regulation system, different frequency difference dead zones are arranged according to different control mode parameters, and a regulation system control mode parameter is selected according to frequency difference swinging change ratio. When a relatively oscillation fault occurs in a power grid to cause that a power plant enters an isolated network, the speed regulation system can accurately act and switch to achieve switching of a control mode and a control parameter, namely, a primary frequency regulation mode control parameter is switched to an isolated network mode control parameter and the isolated network mode control parameter is switched to the primary frequency regulation mode control parameter.