Abstract: The effective ranging of IR photodetection equipments, in addition to the relation with the performance of the equipment itself, is restricted by ambient temperature, relative humidity, visibility ratio and target characteristics etc. To decide whether the effective ranging of IR photodetection equipments is up to standard is now a striking problem in the test-range testing,because the standard conditions are hard to be established in a test range and any real target can be hardly used under test. In this paper, the effective ranging of IR photodetection equipments is calculated by using the traditional formulas calculating the effective ranging, by counting a great number of testing data and by taking an actual measurement for the optical radiation characteristic of some real objects. The important parameters in the computation formulas are quantized. According to the requirement of the defined effective ranging at the standard condition,the effective ranging which should be within reach in the condition of the field test(i.e.the equivalent ranging)is calculated.And with this,it can be decided whether the effective ranging obtained in the actual measurement during the real testing may meet the define requirement.