The invention relates to the technical field of nonlinear load and parameter identification of power systems, in particular to a method for acquiring a load nonlinear function. The method comprises the following steps: at first, utilizing the sampling values of voltage and current to build a high-order differential equation, so that both voltage and current are restrained by the high-order differential equation; then fitting an acquired voltage and current signal; substituting the fitted function into the high-order differential equation to identify the nonlinear function of a load. The method has the benefits that the load equivalent impedance identification precision is improved, and the property of the load with relatively high nonlinearity can be identified. The nonlinear load function can be obtained to bring convenience for learning about the load property of a load in a power grid, facilitate detection of the source causing harmonic pollution in the power grid and be convenient for selecting the optimal harmonic compensation point and determine the harmonic compensation dosage. The power-grid-side change of the power system can be tracked, the real operation condition of the power system can be reflected more relevantly, and accurate equivalent parameter values can be provided for the power system to conduct load flow calculation, voltage stability calculation and other operations.