As an intrinsic characteristic, the shaft-rate modulated electric field of a ship is an important signal source in underwater domain. A measurement system is designed to measure the shaft-rate modulated electric field of a ship. Taking into consideration the pressure bearing, watertightness and other problems, the system is made up of sensors, signal condition circuits, and data acquisition and processing system. After the measurement system is sunk down to the seabed, it is difficult to regulate orientation, attitude and other sensor parameters. It is also hard to control the environment around the system. Therefore, the status information of the seabed is recorded for posterior calibration. Frequency analysis and time-frequency analysis methods are used for signal processing. Experiment results show that the spatial distribution of shaft-rate modulated electric field has a significant regional property and apparent traffic characteristic.