Off-axis parabolic mirror has been widely used in sensor calibration, radiation measurement, infrared target simulation system and so on, because it takes many advantages such as no chromatic-aberration, common used material, broadband, no center block. The irradiance characteristic of the off-axis parabolic collimating mirror was evaluated by both theoretical analysis and simulation study. Both the expression for the irradiance distribution on the exit pupil plane of the off-axis parabolic collimator and the relations between the angle of divergence and the parameters of the collimator such as the focal length of the mirror, the aperture size of the source and the angle of off axis, were theoretically derived based on the radiation transfer model. The irradiance distribution on the exit pupil plane of the collimator was simulated by ray tracing method. The uniformity of the distribution was also analyzed. The method proposed here is useful to increase the collimation and uniformity of the off-axis parabolic mirror.