The internal crack detection of polymer bonded explosive （PBX） is of great significance and engineering application value for its safety and structural integrity evaluation. In order to improve the imaging detection accuracy and image quality of PBX internal crack defects， the curved surface modified water immersion ultrasonic total focusing imaging method was proposed. On this basis， the delay multiply and sum （DMAS） technology was further combined. The ultrasonic imaging detection of the bottom crack defects of the Φ100.0 mm semi-cylindrical PBX was studied by water immersion method， and high-precision imaging characterization and high signal-to-noise ratio imaging of this bottom crack defect of the curved PBX was realized. The experimental results show that the crack defect height measurement error of the traditional TFM imaging algorithm is 12.0%， and the image signal-to-noise ratio is 1.37 dB. The height measurement error of this crack defect after surface correction is 3.6%， and the image signal-to-noise ratio is 2.13 dB. The crack defect height measurement error of the surface correction algorithm combined with DMAS is only 0.4%， and the image signal-to-noise ratio is 5.32 dB.