The maximum detection range equations of the infrared panoramic searching system are investigated, based on the focal plane irradiance distribution, the minimum resolved contrast, detection probability, false alarm probability and the bandwidth. The main factors including the target characteristics, atmosphere background, and the optical, mechanical and electrical characteristics of the systems are analyzed. In the equation of detection range based on the irradiance distribution, the results show that the radiation propagation intensity of target has approximate ascending parabola relation with detection range, atmosphere extinction coefficient has descendent parabola relation with detection range, the system focus has direct proportion with detection range, and the inherent contrast of the target has ascending parabola relation with detection range. Given the definite false alarm probability, the signal noise ratio has approximate descendent parabola relation with detection range, while detector dwell time has approximate direct proportion relation. The system detection distance under the above factors are simulated ,and the results show that the detection range of panoramic visible light searching system can achieve more than 100 km, panoramic infrared searching system can achieve more than 60 km with the target radiation propagation intensity of 70 J/(s·sr), the connatural contrast of more than 4, atmosphere extinction coefficient of less than 0.1 and the system focus of 500 mm.