To quickly search for rare fast radio bursts (FRBs) from massive astronomical observation data, Radio Frequency Interference (RFI) mitigation is one of the crutial and challenging tasks. RFI will produce a large number of false positive candidates, and even submerge real astronomical events. Due to the complexity of the sources and types of RFI, there is currently no universal method to solve this problem. In order to reduce the impact of RFI on FRB search, the RFI in L band observational data of the Nanshan 26 m radio telescope was analyzed and studied. A three-level RFI mitigation procedure was established for the main narrowband RFI and broadband RFI, which effectively alleviated the RFI pollution of observational data. Embedding this procedure into the FRB DDSS (Dispersion Dynamic Spectra Search) pipeline, experimental results show that the detection rate and accuracy of the search algorithm have been further improved. This method provides valuable reference for RFI mitigation of FRB observational data.