Active jamming was one of the most important menaces for radars, and the radar detection range was the groundwork for evaluation of anti-jamming. Based on the analysis of theoretical computation and practical anti-jamming measures, a modified mathematical model for calculating the radar detection range in jamming environment was proposed. The simplified model only needed the calculation of a few modification coefficients, so it was convenient for application. The model could also explain the dynamic change of radar detection range under different anti-jamming measures, thus it was helpful for the evaluation and improvement of the anti-jamming performance. The simulations showed validity of the modified model.