Abstract: The influence of void fraction on dynamic characteristics of pressure drop fluctuation in the gas- liquid two-phase flow in a horizontal pipe is investigated in this paper. By analyzing the relationship between pressure drop and void fraction on the separated flow model, the effect of void fraction on the Mean-Square Deviation (MSD) of pressure drop fluctuation and the correlation between the parameter r (the ratio of MSD of pressure drop fluctuation to the pressure drop of gas-liquid flow) and the void fraction α are obtained. The value of r mainly depends on void fraction when the flow is full developed and under certain flow regime. Experiments are conducted in a horizontal air-water flow facility with a test section of inner diameter 40 mm. The distance between the upper tapping point and lower tapping point of pressure drop is 400 mm. The flow rate of air varies from 0 to 6 m 3·h -1 in accordance with need. Two different water flow rates are used during experiments: 0.37 kg·s -1 and 1.33 kg·s -1 . The pressure drop signal is measured by differential pressure transducer and sampled by computer with a sampling frequency of 200 Hz. The length of every signal sample is 60s and the sampling process was repeated 3 times under each test condition. Experiment results show that the MSD of pressure drop signal corresponds to void fraction in a monotonic relation under certain flow conditions. Two trends of correlation between the MSD of pressure drop fluctuation and void fraction under two different water flow rates are similar when void fraction is lower than 0.5.