Radio Frequency Interference (RFI) mitigation is essential for supporting the science output of Five-hundred-meter Aperture Spherical radio Telescope (FAST) due to its high sensitivity. In order to protect FAST from RFI, an Electromagnetic Compatibility (EMC) study has been carried out and the operation of a Radio Quiet Zone (RQZ) is ongoing. RFI measurements of the telescope instruments and monitoring of the active radio services outside the site have revealed the radiation properties of the RFI sources. Based on the measurement results and theoretical analysis, various EMC methods have been implemented for the telescope to decrease the RFIs. Meanwhile, the main RFI sources in the FAST RQZ, such as mobile stations, broadcast stations and navigation instruments, have been identified, and the technical measures have been adopted to protect the quiet radio environment around the site. The early science outputs of FAST have demonstrated the efficiency of RFI mitigation methods.