Abstract: Modal test analysis is widely used in the structural mechanism studies in the aerospace field and plays a very important role.A method to carry out modal test for vulnerable or precious structure is studied in this paper.This method is named equivalent model method.Modal test can be simplified with the development of the equivalent model.In order to make the equivalent model is equivalent to its prototype,it is necessary to keep the geometrical and stiffness equivalent.The geometrical equivalent is satisfied through the UNIGRAPHIC software at first,the mass of the test model is equal to the prototype.Based on the basic plate theory and structural sensitivity analysis,this paper discussed the theory of equivalent model whose structural material is different from the prototype.This can make sure that the stiffness of test model is equivalent to its prototype.In order to verify this theory,it is applied to the second reflect mirror unit of a remote sensing camera.Through analysis,any frequency and modal shape of the test model of the second reflect mirror unit is equal to its prototype.So if this method is used in the similar condition,satisfactory result can be obtained.This method can be used as a solution to simplify the modal test of aerospace equipment.