Abstract: Objectives: Sea level changes have significant impacts on coastal ecosystems and human survival and development. Monitoring sea level is an essential part of global climate change and marine disaster surveillance. Methods: This study uses GNSS-IR technology to analyze data from 11 frequency bands across four major satellite navigation systems, inverting for long time series of tidal level results from 2018 to 2022 at the Hong Kong HKQT and Seattle SC02 stations, and performs tidal harmonic analysis and estimates long-term change trends. Results: The results indicate that the accuracy of sea level inversion varies among frequency bands. After correcting for dynamic sea level errors, the accuracy is around 10 cm. Different receivers and environments can lead to varying accuracy performances in the same frequency band. Dynamic sea level correction effectively reduces GNSS-IR sea level inversion errors, with an average precision improvement of 20.5% in terms of ubRMSE. Using multi-mode multi-frequency data has enhanced the temporal resolution of sea level inversion, and to some extent, improved the data's accuracy and continuity. Tidal harmonic analysis shows that the amplitude results across 11 frequency bands align well with the tide gauge results, with overall differences less than 1 cm. However, some frequency bands show significant phase discrepancies in certain tidal components, related to the lower temporal resolution of single-frequency sea level inversion results. Long-term trend analysis indicates a sea level rise of approximately 6.0 mm/year near Hong Kong and a sea level decline of about -4.0 mm/year near Seattle, which aligns well with tide gauge results. Conclusions: The findings of this paper not only assess the performance of multi-mode multi-frequency GNSS-IR technology in monitoring sea level changes but also reveal long-term trends in sea level, providing valuable references for marine disaster monitoring and oceanographic research.