Abstract: Closed frogmen wear diving suits, glasses, helmets and flippers, while carrying underwater closed-circuit ventilators to work underwater. In this paper, the echo characteristics of closed frogman are studied through theoretical modeling and measurements. The planar elements method for predicting the echo characteristics of the target on non-rigid surfaces at high-frequencies is locally modified to adapt to the calculation of target echo characteristics of different material combinations under occlusion. A layered medium reflection coefficient model of diving equipment of closed frogman is established. By combining this model with the three-dimensional accurate modeling of frogman geometric shape, the echo characteristics of closed frogman are calculated. The detailed computational images of the 0°～360° target strength of swimming posture, the echo time-angle spectrum, and the distribution of bright spots on frogman surface are given. The lake experiment of a real closed frogman is carried out. Theoretical models and experimental measurements are compared and analyzed. The results show that the closed-circuit scuba bottle and diving suit are the main contributions to echo, and the probability distribution range of the target strength of a closed frogman is -20～-13 dB at 50 kHz and 70 kHz, and -23～-17 dB at 80 kHz.