The measurement accuracy of liquid level is affected by liquid fluctuat ion in a connected pipe measurement system. This paper presented a mathematical model of liquid flow of a threechannel connected pipe by using the knowledge o f hydromechanics. Equations of liquid fluctuation in the connected pipe are deduc ed and characteristics of liquid fluctuation in time domain are obtained through the simulation analysis. An experiment was conducted to testify the theoretical analysis. Experimental results show that the analysis of the liquid fluctuation in connected pipe is correct. The liquid fluctuation added to displacement sign al can be reduced effectively after optimally designing the system.