A novel design method for long-wave infrared (LWIR) fisheye lens was proposed based on Gaussian optics and the third-order aberration theory. The half field of view (FOV) of traditional inversed telephoto wide-angle lens increased from $\pm {70^\circ }$ to $\pm {100^\circ }$ by adding a negative meniscus to pre-group. By using the equidistant projection imaging method, the f-θdistortion of the system was less than 6%. The focal length was 1.97 mm, its relative aperture was 1∶1, the system was telecentric structure in the image space, and its relative illumination of the marginal FOV achieved 0.8. A five-piece aspheric design was adopted to increase the aperture and remove the off-axis aberrations. The imaging quality is near-diffraction-limited. The modulation transfer function value was above 0.37@42 lp/mm. The imaging quality of the system was stable at a wide range of temperatures with the mechanically passive athermal method. The results have a certain reference for LWIR fisheye optical system design.