<p>This paper presents an ultrasonic detection method tailored for evaluating impact damage in carbon fiber reinforced polymer (CFRP) plates, which fundamentally relies on the interaction between a bounded ultrasonic beam and a CFRP plate immersed in fluid. Initially, the non-specular reflection sound field expression for a bounded ultrasonic beam obliquely incident on the CFRP plate is derived. Subsequently, finite element analysis reveals the correlation between the signals detected by the specular receiver and the impact damage status under different parameter combinations. It is found that under the optimal parameter combination and the A0 critical angle, even a slight degradation in the elastic constants of the CFRP plate induces a monotonic and significant increase in the change rate of the specular reflection coefficient (SRC). In the experimental phase, tests conducted on the water-CFRP plate-water structure validate the aforementioned finite element simulation results. The experimental data consistently demonstrate that at the A0 critical angle, the change rate of SRC increases monotonically with increasing impact energy—confirming the effectiveness of the bounded ultrasonic beam-based non-specular reflection method at the A0 critical angle for assessing impact damage in CFRP plates.</p>