In applications of grid-tied inverters with LCL filter, the digital delay and lumped disturbance may deteriorate the performance, even result in instability. For the purpose of alleviating the impact of computational delay and improve the ability against disturbance, this paper presents a composite controller based on the disturbance and state observer, combining with a PI controller and a full state-feedback which plays a role of the active damping. The proposed strategy utilizes a state observer as a state predictor to eliminate the influence of computational delay, and a disturbance observer to suppress the model uncertainty and unmodeled dynamics. Therefore, an excellent transient response of system can be achieved. Simulation results demonstrate that the designed system has the characteristics of superior disturbance rejection, desired transient and steady-state performances.