Interferometric imaging radar altimeter (InIRA) is a spaceborne sensor which can measure the sea surface height (SSH) with high precision and wide swath. The error of interferometric baseline incline angle is one of the main factors affecting the precision of SSH measurement. However, the accuracy of direct measurement of incline angle usually do not meet the demands. Accurate estimation of incline angle is necessary for InIRA. According to the characteristics of double side-looking configuration, this paper proposed a new method to estimate the baseline incline angle for double side-looking InIRA. And simulation is presented to verify the proposed method.