Abstract: The navigational radioactivity measurement in seawater or on seafloor sediments is an efficient and economic method for geophysical exploration. This detector can be applied in fields such as marine radioactive pollution monitoring, marine geology, mineral exploration and the alike. The detector tested in this paper consists of sodium iodide, pre-amplifier, digitizing spectrometer and the decompression and shock retarding materials. The bundled software includes the basic functions for geophysical investigation and it is user-friendly. On the basis of systematic tests including stability test, resolution test and calibration source test in the laboratory, the fixed point tests at wharf, on the sea and sailing measurement in the sea, it shows that the detector has the advantages of steady performance, high energy resolution and high sensitivity. Furthermore, the tests also show that the detector is able to distinguish the radioactivity of many substances. It means that we can classify the substances from the measurements. This is meaningful for further study of classification of marine sediments.