It is quite essential to calculate wave forces for the design and stability of breakwater.With the aid of physical model test,we study the distribution and influence factors and calculation method of wave trough pressure on the large diameter cylinder breakwater with different spacing under irregular wave,in view of the imperfection of design and calculation method of wave trough pressure on the cylinder breakwater.The results show that the transverse distribution of wave trough pressure on the large diameter cylinder structure is related to the spacing of the cylinder under the action of the waves.The longitudinal distribution appears that wave trough pressure is linearly enhanced and then linearly dropped off with the increase of water depth,and the position of the maximum wave trough pressure arises about the depth of wave height below static water level.The relative spacing,relative depth and wave steepness of the cylinder are great influence on the wave trough pressure on the large diameter cylinder breakwater with different spacing.The wave trough pressure calculation formula of the cylinder breakwater is provided with the reduction coefficient on the basis of the theoretical wave trough pressure formula of the vertical wall structure.