Based on complex Gaussian-function expansion method, the transmitting wave is decomposed into a series of Gaussian beam components. ABCD ray-matrix theory combined with Rytov approximation is used to derive the expressions of the mean intensity and the backscattering enhancement coefficient of the reflected wave from a finite circular mirror in the atmospheric turbulence, which reveals the existence of the oscillation zone. The degree of coherence function and coherence length are calculated and the results prove that the turbulence in the oscillation zone has a larger effect on the wave front.