In the existing vehicle assisted driving systems, a visible imaging system is difficult to be effectively applied to night, rain, fog, haze and other bad weather conditions with low illumination. An infrared imaging system can effectively overcome the above shortcomings, but the traditional vehicle infrared system has the shortcomings of a small field of view and large blind area. The vehicle application of the ultra-field infrared system is studied in this paper, and a demonstration method of key parameters is proposed for ultra-field infrared system application by utilizing the joint constraints based on safe distance and imaging size. Specifically, the isometric projection imaging law is firstly analyzed; then, the constraint models of safety distance and imaging size are established. Finally, its feasibility is verified by the nearest detection distance corresponding to the limit speed and the longest discovery distance under the constraint of the two-dimensional Johnson criterion, and its key parameters are calculated. The ratio of the lens focal length to pixel size range of the ultra-field infrared vehicle mounted system shall be within the range of[276.1 521.3] through theoretical demonstration and analysis. The system experiments designed the ultra-field infrared imaging system with a horizontal field angle of 140° is designed in the system experiments. The detection distance of the infrared imaging system is more than 130 m, which can meet the requirement of discovery distance for auxiliary driving.