A symmetrical detection configuration was developed to detect inner surface defect in a hollow cylinder by laser-induced ultrasonic. In this method, circumferential waves were detected at two points on the outer surface with ±90° relative to the laser source and their difference was used to obtain the differential circumferential wave. Finite element method was used to analyze the influence of inner surface defect’s position and size on the differential circumferential wave. The results show that the relative energy of differential circumferential wave and the emerging time of the bipolar waveform strongly depend on the defect’s size and the source-defect angle. As a result, the position of defect can be determined by the reach time of the bipolar waveform and the size can be evaluated by the relative energy of differential circumferential wave.