In structure-equipment system, there are complex dynamic interactions between structures and equipments. A fast discriminant condition whether the interaction will increase or decrease dynamic response of the structure is achieved in this paper. When the load frequency lies between the basic frequencies of the combined system, it decreases the absolute response of the structure and vice versa. In engineering project, the seismic load can be expanded based on ground characteristic frequency by Fourier transform, so our conclusion can be extended to seismic design for combined systems. Finite element method simulation for a chemical workshop further verifies the discriminant condition.