As an expendable profiler in marine environment, XCP (Expendable Current Profiler) is mainly to measure current speed, direction and temperature. The paper is to research dynamic sinking data transmission technique for XCP. Data transmission medium is a varnished wire with diameter of 0.1mm, length of 1,800m and two parallel wires. Transmission characteristics of varnished wires can be tested by experiment and computation. The paper is to research and design a kind of base band transmission system and analyze its characteristics and performance. By utilizing digital base band signaling transmission technique, indoor and marine testing results show that designed data transmission system can reach 1,800m on varnished wires with diameter of 0.1mm if XCP probe sinks dynamically by speed of 4m/s and transmission speed is 2,400bps.