A low frequency capacitive micromachined ultrasonic transducer (CMUT) array is presented to detect underwater. Based on the analysis of CMUT array structure and work principle, the directivity expressions of CMUT element and the one-dimensional array is established combined with the array composite structure, the geometric parameters are determined in view of the influence on the directivity of CMUT array. According to the application requirement of CMUT underwater detection, the paper studies underwater packaging methods of CMUT array complete the CMUT array water sealing through package structure design and packaging material selection. At the same time, the paper develops a CMUT array performance test scheme and sets up the corresponding test and detection system. The test results show the low-frequency CMUT has better detecting directivity, the -3dB main beam width and maximum sidelobe level for CMUT array are respectively 5°and 13.5 dB, and the CMUT array can realize clear underwater detection within the scope of 1m. The experiment results show the proposed design and encapsulation method is reasonable，and provide the design basis for improving the underwater detection performance of CMUT array.