This paper focuses on the temperature effect and its solution. Analysis is made concerning temperature effect on three factors: refractive index of optical elements, curvature radius and center thickness of optical elements, and the space distance between optical elements, and their relation to temperature variation are as well discussed. Athermalisation is introduced to eliminate the influence of temperature in optical systems, and the advantage and the disadvantage of the three athermalisations used currently are compared. An infrared optical system is taken for example. In the design process, the mechanical active athermalisation is used for eliminating the influence of the temperature change . And the image quality comparison with/without athermalisation is made. It is concluded that the influence of temperature effect could be eliminated so that the high image quality could be realized in a wide temperature range.