Abstract: The extrapolation technique is introduced, and an absorber effect evaluation method is proposed based on the geometrical optics. The method can effectively simulate the reflected wave from the absorber, and by using the extrapolation technique twice, the effect on antenna gain results can be evaluated. The method has been verified in the new extraploation range of, and NIM (National Institute of Metrology, China) the results agree well with the prediction. The uncertainty due to the absorber effect can therefore be evaluated and the proposed method has already been applied in the extrapolation ranges at NIM and NPL(National Physical Laboratory, UK). The application of the method is not limited to extrapolation ranges, and it can be easily extended to be applied in other antenna measurement systems located in the chamber.