The detection effectiveness of the airborne early warning radar refers to the degree to which the early warning radar accomplishes its mission of early warning and detection. The radar's detection effectiveness tends to be affected by its detection range as well as its detection probability. A method of evaluating airborne early warning radar's detection effectiveness is given based on probability analysis method. A mathematic model is first established between the radar detection probability and its maximum detection range. Then, the curve of detection probability with detection range is normalized and the probability density function of detection probability is calculated. After that, the probability distribution function is acquired through integrating the probability density function of detection probability, the function has actual physical meaning and it reflects the detection effectiveness of the early warning radar. Thus, the detection range is quantitatively converted into detection effectiveness, and the curve is drawn for the detection effectiveness that varies with the change in range. And finally, by taking E2C and E3B early warning radars for example, an analysis is done aimed at the targets of Swerling I and Swerling II through comparison to verify the validity of the proposed method.