Generally speaking, Radar Cross Section (RCS), which is an important indicator to value the electromagnetic scattering characteristic of a target, has been paid more attention as time goes by since WWI. As we know, when RCS measurement is carried out, the accuracy of RCS measurement results always depends on the testing site and the system, and if the target is too large or the frequency is too high, a cooperative and vast measuring area or a costly compact range facility will be needed. As the testing site sometimes is not large enough for some cases, so the radiation pattern of the transmitting-receiving antenna can’t be dealt as the far filed pattern. To solve these problems, this paper presents a new method of RCS measurement basing on SAR imaging by a scanner in near field. And also we employ a new SAR imaging method instead of the traditional ISAR imaging method to transform the electromagnetic scattering near field data of target to the far field RCS. That’s saying, the near field data is collected by a portable 1-D scanner SAR imaging system, with the new method, we can obtain the far field by the transformation. Furthermore, in order to get the accurate far-field RCS data of the target, we use a new method to correct the effects of transmitting-receiving antenna’s radiation pattern and the difference of the distance from scattering center of the antenna to target in near-field imaging measurement, which we usually call corrected-imaging method. The compared results of theoretical simulation and experimental data show that RCS obtained by the proposed method is satisfied the condition and can be used widely.