Abstract: According to the structure of the steel mobile pipeline, a suit of liquid holdup detecting device with two capacitance probes is designed and manufactured based on the different permittivity values between the water and the gas. The device was applied to measure the liquid-holdup during mobile pipeline draining. Some results are found. First, the device is sensitive to the variation of liquid-holdup, so it can be used to measure the liquid-holdup during the draining online continuously, and the gas-liquid two phase flow pattern could also be judged by analyzing the liquid holdup variation. In addition, the liquid-holdup in the draining process is closely related to the gas pressure and flow rate. The higher the flow rate is, the more likely the slug flow occurs, the liquid-holdup is relatively low, and the gas percentage in the pipe is relatively high. On the contrary, the higher the pressure is, the lower the frequency and quantity of the slug flow occurrence are, and the liquid-holdup in the slug flow is relatively high. It shows that high pressure can increase the average liquid-holdup of the pipe, which is beneficial for keeping the pump working continuously and saving the time of draining.