The monitoring of offshore oil spills is becoming increasingly important, but the various methods available are mostly used in the monitoring of large-scale oil spills. In view of the monitoring of underwater small oil spills and initial oil spills, this paper proposes to use the four pins at the micro USB end of the USB data cable as electrodes to measure the impedance, and the mutation of the impedance indicates the detection of oil droplets, that is, the monitoring of seawater oil spills is realized. The standard deviation for measuring 400 impedances in salt water is 0. 058 Ω. The three types of oil (gasoline, diesel and lubricating oil) and salt water were distinguished, and the impedance change from salt water to oil increased by at least 418% and the response time was less than 0. 19 seconds. This method can realize the high-sensitive and rapid monitoring of oil spills in seawater, and has a broad application prospect in the future.