Through studying visual perception of asymmetric distortion of stereoscopic image, a quality assessment method for asymmetric distortion of stereoscopic image based on Wavelet Image Fusion (WIF) is proposed. Firstly, the block disparity value is computed by fixed block marching method. Then, the original and distorted fusion images are obtained by WIF according to original block disparity value and stereoscopic visual perception. Finally, the stereoscopic image quality is objectively assessed by calculating the distances between directional projection of original and distorted fusion images. Experimental results show that the proposed objective method achieves consistent stereoscopic image quality evaluation results with subjective assessment for various types of distortions, especially shows its effectiveness for assessing stereoscopic image with cross-distortion.