Compound function means that under the condition of x→y (mapping) (where X is the independent variable and Y is the dependent variable), with Y as the independent variable, Z as the dependent variable and y→z (mapping), the corresponding relationship is y=f(x) and Z = f (y) = f (x), thus forming simple and complex compound functions. Method of substitution, the most outstanding performance of composite function, transforms the range of a function into the domain of definition, brings it into the corresponding function, finds the domain of definition, and finds the domain of independent variable ("meta") according to the original hypothesis. F(x) and f(t) are the role exchange between dependent variable and independent variable.