Abstract: In order to improve elastic constants ultrasonic characterization accuracy for unidirectional carbon fiber reinforced plastic (CFRP) plates, experimental measurement and numerical simulation were conducted based on back reflection ultrasonic immersion technique. A single-axis rotation testing setup was developed firstly, and the delay time measurements and ultrasonic velocity calculations at different angles were realized by combining cross-correlation signal processing method. Then, the five independent elastic constants were obtained by the inversion of ultrasonic velocity data using simulated annealing optimization algorithm. Finally, the experimental measurement results were verified by theoretical calculation and CIVA software simulation. The results indicate that the ultrasonic velocities of quasi-longitudinal wave and quasi-transverse wave varied with incident angle in 1-3 plane, which is observably distinct from that of 1-2 plane, thus shows that the material is transversely isotropic. The independent elastic constants are 12.88, 6.39, 126.83, 4.85, 2.97 GPa, respectively. The deviations for ultrasonic velocities of theoretical calculation, CIVA software simulation and experimental measurement results are below 6.4 m/s, the fluctuation of experimental measurement data is below 0.33%, and the corresponding errors of elastic constants are below 0.47%. The methods concerned are also valid for the testing of elastic constants for other orthotropic media.