The inundation state is one of the main factors affecting the development of river floodplain wetlands, and the water inundate frequency (WIF) is a commonly used characterisation tool. This study combines remote sensing data with ground hydrological observation data to propose the water inundate guarantee rate (WIGR) as a new tool to characterise the inundation probability in river floodplain areas. It further conducts a comparative study between WIGR and WIF in the Yimin River, located in China's Inner Mongolia Autonomous Region. The results show that the inundation state maps of the study area are generally consistent using WIF and WIGR. However, there are observed differences in specific regions. In regions with very low assurance rates, it covers a larger area of 2.90 km² using WIGR relative to WIF. Meanwhile, The area of other low, medium, high and very high areas is relatively lower. The reason for the difference is that the selected 52 phase remote sensing data corresponds to a low flow guarantee rate, which is generally medium and high. Further, the flow guarantee rate introduced in the WIGR analysis process plays a corrective role in the final analysis results. Compared with WIF, the WIGR algorithm is systematic, continuous, and has certain statistical significance, which has higher scientific credibility and accuracy. Specifically, the simple operation of the WIGR algorithm shows great potential to be applied to other related studies. Nevertheless, further improvement of the WIGR algorithm should focus on the influence of the atmospheric environment on remote sensing data and the acquisition of hydrological data. The WIGR proposed in this study can provide technical support for the conservation of river floodplain wetlands and open new ideas for studying river floodplain wetland ecosystems.