Adaptive imaging systems have been developed to compensate for distortion introduced by atmospheric turbulence. The performance of its imaging quality can be evaluated by the Strehl ratio, but this does not directly quantify the detecting ability of an adaptively correcting telescope. Combining the normally detecting SNR and the telescope Strehl ratio, a new method evaluating the detecting ability of an adaptive astronomical telescope is put forward, which can give a quantified value. The new method is simulated on a computer, the simulation results indicated that the detecting ability of an adaptively correcting a 2-m telescopes improves 2 visual magnitudes. The effects of different atmospheric coherence length on detecting ability also can be quantitatively studied through the new method, which can give a scientific basis for the optimization of the design of the system and the development of implementations.