Abstract: To understand the electromagnetic environment surrounding the Five-hundred-meters Aperture Spherical radio Telescope (FAST) before and after its operation, electromagnetic signals in the main observation frequency bands of FAST were observed using standard radio signal observation methods. Based on the observation results, the distribution characteristics of electromagnetic signals in the vicinity of the FAST were analyzed as follows: during the commissioning phase of the FAST, the dominant signals included broadcast signals in the 87−108 MHz band, aeronautical mobile signals in the 117.975−137 MHz band, and fixed mobile signals in the 806−960 MHz band, with the highest frequency occupancy found between 900−1200 MHz, reaching 1.8%. After FAST became operational, new broadcast signals emerged in the 485−566 MHz and 614−640 MHz bands. Additionally, China Broadnet 5G public mobile communication signals have been added in the 702−798 MHz frequency band, and new fixed and mobile signals have been introduced in the 806−960 MHz frequency band. Meanwhile, affected by the increased deployment density of 2G base stations and the integration of 3G and 4G signals, the frequency occupancy significantly increased in the 1 710−1 930 MHz range, with the overall occupancy from 1 200−2 000 MHz rising to 6.78%. Compared to the “Interference Protection Requirements for the 500-meter Aperture Spherical radio Telescope (FAST),” not all measured signal spectral power flux densities met the required thresholds. These observations were conducted concurrently with the FAST commissioning and operational testing, including signals from telemetry and communications. This analysis provides important reference insights for improving the electromagnetic environment around the FAST and guiding its operation.