Abstract: In order to expand the stability of substructure testing to nonlinear structures, the unconditional stable energy-conserving time integration algorithm is implemented in the substructure test and a substructure testing method based on the algorithm, called energy-conserving substructure testing method, is developed. The equivalent force control method is used to solve implicit difference equations. The computation of equivalent force command with delay compensation is presented using the 3rd-order polynomial compensation for real-time energy-conserving substructure tests. Real-time energy-conserving substructure tests were conducted with nonlinear buckling-restrained brace specimen. The results show that the energy-conserving substructure testing method is successful for nonlinear structures. The tests validate equivalent force control method for nonlinear implicit difference equation. The real-time substructure tests show that the 3rd-order polynomial compensation can effectively compensate the actuator delay.