Slug flow was a flow of long bubbles and liquid slugs alternating in space and time，and under flowing conditions，it is intermittent and unstable.The fluctuation characteristics of liquid holdup，pressure and differential pressure of slug flow were investigated in a 40 m long，50 mm I.D.horizontal pipeline.By statistical analysis，it was found that the probability density distribution of the liquid holdup was bimodal distribution.The high liquid holdup peak was in correspondence with liquid holdup of the slug body and the low liquid holdup peak with liquid holdup of the film.The liquid holdup which was in correspondence with the respective peaks of probability density function was consistent with the mean liquid holdup of smooth stratified film and liquid slug.Moreover，the distribution of pressure was unimodal distribution or bimodal distribution，depending on the number of slug units，and the differential pressure distribution was unimodal distribution.These characteristics provide dependable slug identities for flow pattern identification.