The invention relates to a long wave infrared 5mm optical passive heat dissipation lens, which comprises a main lens barrel and an optical system arranged inside the main lens barrel wherein the optical system is provided in sequence with a negative meniscus lens A, a first double convex lens B, a fixed aperture C, and a second double convex lens D. The air gap between the negative meniscus lens A and the double convex lens B is 1.52mm; the air gap between the first double convex lens B and the fixed aperture C is 5.43 mm; and the air gap between the fixed aperture and the second double convex lens D is 2.13 mm. According to the invention, the lens has a simple and compact structure, which can overcome the influence of temperature on the imaging performance of infrared lens. Through the proper distribution of the focal power of the lenses and through the use of the infrared sulfur glass with low temperature refractive index coefficient (dn/dt) characteristics, it is possible to achieve self-adaptive optical passive heat dissipation of temperature so as to enable an infrared optical system to keep good imaging quality in a larger temperature scope and to reduce the processing difficulty and cost of lenses.