Abstract: In deep ocean,to detect the underwater target located in the first shadow zone,the bottom bounce mode is often used by the low frequency active sonar.However,the range estimation error occurs due to the variable sound speed which produces a bending acoustic ray path from the sonar to the target.To reduce the active ranging error,the method using the effective sound speed can be used.However,this method faces a heavy computation burden which comes from the calculation of the time delay-effective sound speed pairs at each position in the bottom bouncing area.To solve the problem above,an improved effective sound speed estimation method based on the interference structure of the sound field is proposed.We show that the sound field fluctuation in the bottom bouncing area is due to the energy fluctuation of the sound rays having different grazing angles.And a quantitative relationship between the detectable areas and the sound rays with high energies is calculated based on the sound ray interference theory.The time delay-effective sound speed pairs in the boundary location of the detectable areas can be calculated according to this relationship.And hence,time delay-effective sound speed pairs of all the positions can be obtained by the linear fitting.The simulation results show that the improved method can obtain a similar range estimation error to the conventional effective sound speed estimation method,while the former has a much low computation burden which is useful for the real-time calculation.