The principle and procedures of trilinear pendulum which is used to test the moment of inertia of complex components are elaborated. The equations of moment of inertia are presented to obtain the product of inertia. By analyzing the equations deduction of trilinear pendulum, the simplified error can be 0.28%. The parameters of the radius of upper and down round plates, the mass of objects, the cycle of swinging can be calculated through the method of trilinear pendulum, but the errors are greater and the aggregated measuring errors of the parameters can reach 5% or above. Based on the detailed analysis and provement, the methods of decreasing errors are proposed. 