Mathematics foundation is introduced,and the key points are Marchenko-Pastur Law and Single-ring Law.General method of applying large dimensional random matrices theory to condition assessment is conducted,including application condition,matrices construction method,and assessment indexes.Then the specific programing steps of single-ring law are discussed.Finally,two application scenarios are taken as examples,respectively are electrical equipment condition assessment and power gird condition assessment.The simulation results indicate that large dimensional random matrices theory is able to be applied to assess the status of a complex system,the health status of electrical equipment and the fault operation status of power grids shall be reflected through the assessment indexes.