A passive ranging method for IRST systems aimed at uniform velocity point targets was presented. Target model has been built firstly, then the range equations set based on this model were developed. The peculiarity of this mothed is that the velocities, ranges and radiant intensities can be worked out, without targets database and supposing the parameters of targets just like velocities, only with the angles and radiations of the targets measurd by IRST systems. The computer simulation verifies the validity of the method.