This paper presents a new active control strategy based on linear matrix inequality (LMI) and genetic algorithm (GA) for the structural systems with nonlinear uncertainties and exogenous disturbances. Based on structural dynamics theory, the nonlinear uncertain structure system state-space model is established. Then, based on all-order H∞ control and GA, the controller of minimum order is obtained by searching the object function globally. The concerning genetic algorithm adopts float coding, stochastic tournament, elitist model, linear crossover and uniform mutation. Finally, a three-degree-of-freedom building model subjected to EI Centro earthquake is considered using this method and the simulation results show that the provided controller has almost the same control effect as all-order H∞ controller.