In order to further confirm the relations between the size of internal defect and the measured ultrasonic signals and understand the interaction mechanism between the ultrasonic wave and inside defect, this paper has constructed the two-dimensional (2D) plane strain finite element model of the Al plate with the inside defect to simulate the scattering process of the ultrasonic wave generated by the pulsed laser in the Al specimen. The dependence of the normalized amplitude of the defect signal on the defect diameter has been calculated in details, and the ultrasonic reflectance by the cylindrical shaped internal defect was evaluated in details using the numerical results. It is demonstrated that the laser ultrasonic technique has the ability to detect the sub-millimeter-scale internal defects in industrial components and engineering structures.