In this paper, in view of the on-line inspection of the thickness of steel plate and its surface defects, a quality inspection system for appearance of steel is designed. The moire image on the surface of the steel plate is collected by a linear CCD camera. The fringe image is transformed by wavelet transform, and the phase information of wavelet transform coefficients corresponding to the wavelet ridge is extracted to reconstruct the 3D profile. Experimental results show that the thickness measurement precision of the steel plate is 0.08 mm and the measurement accuracy of the surface defect is 0.2 mm. The online detection speed is 6 m/s. The on-line detection of thickness and surface defect of steel plate can be realized.