Tension leg platform(TLP)is a typical compliant offshore structure for oil/gas resources exploitation.In the design process,the prediction of the free surface elevation is of great importance for the determination of the air gap.So far,the existing researches for predicting the air gap of the TLPs focus on the supporting columns while pay little attention to the horizontal pontoons.For the second order diffraction problem or long incident wave condition,the velocity potential decays slowly with water depth and the effect of pontoons should not be neglected.Herein the effect of pontoons on the diffracted wave field in the vicinity of a TLP platform is investigated in this study.The diffraction of regular waves by a square array of truncated cylinders and a whole TLP structure is studied in detail by using both the linear and the second-order diffraction theory.Numerical calculation is performed for the free surface elevation and wave run up.Numerical results show that the near-trapping phenomenon can occur inside the TLP and leads to significantly increased wave height.To study the effect of pontoons on the free surface elevation comparisons are carried out between the results of these two structures with and without pontoons.It is found that pontoons have an appreciable effect on the diffracted wave field for long incident regular waves and increase the largest response notably when the near-trapping phenomenon occurs at the second-order. Tension leg platform (TLP) is a typical compliant offshore structure for oil / gas resources exploitation. In the design process, the prediction of the free surface elevation is of great importance for the determination of the air gap. So far, the existing researches for predicting the air gap of the TLPs focus on the supporting columns while pay little attention to the horizontal pontoons. For the second order diffraction problem or long incident wave condition, the velocity potential decays slowly with water depth and the effect of pontoons should not be neglected . Herein the effect of pontoons on the diffracted wave field in the vicinity of a TLP platform is investigated in this study. The diffraction of regular waves by a square array of truncated cylinders and a whole TLP structure is studied in detail by using both both linear and the second-order diffraction theory. Numerical calculation is performed for the free surface elevation and wave run up. Numerical results show that the near-trapping phenomenon can occur inside the TLP and leads to significantly increased wave height. To study the effect of pontoons on the free surface elevation comparisons are carried out between the the results of these two structures with and without pontoons. It is found that pontoons have an appreciable effect on the diffracted wave field for long incident regular waves and increase the largest response notably when the near-trapping phenomenon occurs at the second-order.