In view of the limitations of the existing polar ice acoustic detection technology in terms of detection bandwidth and deployment convenience, this article focuses on the design and testing of the coupling cavity part of a cross-ice underwater acoustic signal pickup device. The device converts elastic waves in the ice into pressure waves in the coupling fluid through a coupling chamber, a flexible shell with an acoustic-solid coupling fluid, and receives the sound pressure signal through a hydrophone immersed in the coupling fluid. Firstly, the material of the coupling cavity is selected through theoretical analysis. Then, the geometric parameters of the coupling cavity are explored by simulation software simulation. Finally, the pool experiment and low-temperature condition test are carried out, and the influence of the geometric parameters of the coupling cavity on the trans-ice detection effect and the reliability of the device is further determined. The test results show that the thickness of the sidewall of the coupling cavity is an important parameter affecting the cross-ice underwater acoustic signal pickup effect and low-temperature adaptability of the device.