Under the condition of time variant RFI, the limitation of training sample size causes great performance degradation of the conventional Radio Frequency Interference (RFI) suppression algorithm based on eigen-subspace projection (ESP) method. A novel RFI suppression method using iterative adaptive approach (IAA) and orthogonal subspace projection (OSP) method is proposed for synthetic aperture radar (SAR). Dispensing with parametric search and model order estimation, the proposed method estimates the RFI power spectrum adaptively and iteratively, utilizing few training samples and filtering the RFI based on the OSP method. Both the simulation and experimental results are provided to illustrate the performance of the proposed method.