In order to realize the athermalisation of infrared optical system, an finite element analysis (FEA) model is established to analysize the contact stress of the lens and sensitivity of the structure. First, based on the athermal requirements of the optical system, the concept of the flexible pressure ring is introduced. Then, thermal deformation analysis is made on the flexible elements. After that, we design the flexible elements based on the sensitivity analysis, which gets that the flexible elements can reduce the contact stress of the lens and makes the choice of the lens sear materials more easily. Finally, the flexible elements is introduced to the triplet system. The simulation results indicate that the modulation transfer function (MTF) of the triplet system is above 0.5 under the use of flexible elements when the spacial frequency of MTF is 10 lp/mm. The defocus of the ordinary pressure ring under 60 ℃ and -20 ℃ are 4.765 mm and -6.312 mm respectively while the flexible are 1.261 mm and -1.563 mm respectively. The use of the flexible elements can meet the athermal requirements of the infrared system.