The invention discloses a multivariable fitting-based parameter optimization modeling method for a generator speed regulating system. The method comprises the following steps: firstly, by a multivariable fitting method, fitting the relationship between three performance indexes, namely percent overshoot, rise time and regulating time, of primary frequency modulation of a generator and four control parameters, namely unit regulation power, a hydraulic servo-motor time constant, a turbine steam time constant and a generator inertia time constant; with the obtained multivariable fitting formula of the primary frequency modulation as an equality constraint, control parameter value ranges and other conditions as inequality constraints and the optimal primary frequency modulation performance of the fine speed regulating system as a final target, searching the speed regulating system parameter with the optimal primary frequency modulation performance by a multi-target and multi-constraint nonlinear optimization method; and building an optimized generator speed regulating system model.