According to the theory of passive millimeter wave detection, the theoretical formulas of output signal, detection probability and target area of passive millimeter wave detection are given. The curves of detection probability and signal-to-noise ratio are presented. The approximate calculation formulas for calculating the output signals from a passive millimeter wave sensor, detection probability and target area in an approximately linear range are derived. The formulas are used to explain a coincidence phenomenon in a certain test of passive millimeter wave detection successfully. The conclusion reached is of a reference value to the related passive millimeter wave detection.