The liquid accumulation in long distance coal-bed-methane gathering and transportation pipeline will hinder the effective gas transport and accelerate pipeline corrosion. Liquid level measurement techniques, which are a crucial step to overcome these liquid-accumulation-related problems, are mostly either invasive or inaccurate due to the fluctuation of liquid phase. Concerning the limitation of the traditional level measurement technique, a clamp-on liquid level measurement method using ultrasonic Lamb waves is proposed. According to the dispersion curve in the circular pipe, the optimum operating mode and excitation parameters are determined. During experiments, the nonlinear relationship between vertical pipeline liquid level and Lamb wave amplitude is obtained. Meanwhile, the ability of identifying liquid level changes is improved by the algorithm of denoising and filtering the received signal. Based on the results, the proposed measurement can be used for liquid level measurement and provides a potential of flow condition measurement of gathering lines.