Abstract: By using the coding signal excitation and pulse compression receiver,pulse compression technique manages to increase the time resolution and signal-to-noise ratio in ultrasonic detection.However,the relative small time bandwidth product limited by the transducer bandwidth,leads to a poor compression result.The multi-frequency pulse compression method based on ultrasonic array is studied.The array consists of the elements with different center frequencies and bandwidths,respectively,thus it has a larger bandwidth than a single-element transducer.A compression-superimposition-envelope method is proposed,with the research of technical solution to improve time resolution and mainlobe-to-sidelobe ratio.The reasonable design and optimization of ultrasonic array are given,to achieve the multi-frequency pulse compression result of great enhancement on time resolution,and good suppression on the periodic sidelobe resulting from the superposition.Theories and experiments show that the time resolution performance enhancement brought by the compression-superimposition-envelope method is robust to the roughness within the element bandwidth,as well as the improvement on the mainlobe-to-sidelobe ratio.