Abstract: In the process of measuring the RCS of target using RCS measuring radar, in order to get exact RCS value, calibration must be carried out. The calibration of antenna system can be carried out using a calibration hung under a balloon, and the calibration of the receiver can be carried out using a standard signal generator. In the calibration process, the nonideal environment, instruments and methods can bring error to the result. The RCS of the balloon, the machining error of the calibration sphere, the precision of aiming the radar at the calibration sphere are some of the sources that bring non-ignorable error.