In an adaptive optical system, the object tracking error primally censists of the tracking error induced by the incompletely compensated atmospheric turbulence and the telescope vibration and the tilt noise error. A new method with the use of closed-loop residual tilt data is proposed to evaluate the tilt noise error, the tracking error due to the atmospheric turbulence and the vibration of the telescope,and the residual tracking error. This method is used to analyze the fine tracking loop performance of the 61-element adaptive optical system.