The test device of water film flow outside the horizontal tubes for evaporative condenser was set up to investigate the water film flow pattern outside the horizontal tubes and the characteristic length of water film flowing.Experiments were performed on a vertical row of horizontal tubes with stainless steel sheet between them.It is discovered by observations that water film gets into turbulent flow strikingly when the expansion angle of indicating liquid is less than 30° and the film goes into laminar flow obviously when the angle is over 50°.The changes of water film flow pattern outside the horizontal tube with the diameter and flow rate were studied experimentally.The results show that the critical Reynolds number threshold of water film outside the tube is 15 for laminar flow，and 25 for turbulent flow.With the increase of flow rate，the flow pattern of the film outside the larger diameter tube gets into the turbulent state earlier than the smaller one.The characteristic length of water film flow Reynolds number is the same order of magnitude as the water film thickness.The thicker the water film，the greater the characteristic length

and the larger diameter the tube，the greater the characteristic length.