To solve the problem that the traditional transponder systems are susceptible to strong-noise interference and the transmission delay accuracy is not high when simulating underwater targets at different distances， a coarse-fine two-stage arrival time estimation method is proposed， and a reliable transponder system with high precision delay is developed. The system is mainly composed of the upper system， which collects and processes signals， and the lower system， which amplifies and transducers. The upper system uses multistage spectrum analysis， and its peak ratio is greater than the set threshold for rough estimation，and the signal wave headis preliminarily obtained. Then， the double threshold detection algorithm based on the energy entropy ratio is used toacquire a more accurate arrival time by fine estimation.The problem of inaccurate delay is solved. Finally， the lower system receives the response signal transmitted by the upper computer through TCP and responds to the echo signal at a predetermined time. Lake trails show that the delay error of the system is less than 40 ms， and the system has the advantage of high-precision forwarding delay and has good robustness under strong-noiseinterference.