The infrared transmission of ground object and its background were discussed. A new passive distance measuring method was deduced based on the irradiance errors between the ground object and its background detected. By the above-mentioned new method, the formula of distance measuring and the formula of distance resolving power were formed. The relationship between the irradiance errors and the distance, and the relationship between the distances resolving power and the distance under different weather conditions were calculated. The calculation result has proved that the method can be applied to the distance measurement of ground objects.