Vegetation plays a vital role in riverine ecosystems. However, the height of Poaceae vegetation on floodplains encountered continuous adjustments under the combined effects of climate change and anthropogenic activities. These adjustments influence the physical habitat and flood risk management within fluvial systems. Therefore, a long-term and large-scale surveillance of floodplain vegetation height is crucial. In recent decades, advancement in spaceborne Light Detection and Ranging (LiDAR) technology has made it feasible to obtain the vegetation height. Here, we integrated the new-generation spaceborne LiDAR system GEDI and Sentinel-2 data to construct a XGBoost-based vegetation height model specified for floodplain poaceous vegetations by adopting phenological metrics, accumulated temperature, and photosynthetically active radiation. Meanwhile, a flood inundation detection model was constructed based on the Attetion-UNet algorithm. Taking the floodplains in the Middle Yangtze River as a representative example, the applicability of spaceborne LiDAR technology in obtaining poaceous vegetation height was examined. Furthermore, the contribution of each indicator to the model and the vegetation height dynamics in response to different inundation patterns was analysed. The main findings are as follows: (1) spaceborne LiDAR system GEDI demonstrates a capability for accurately detecting the Poaceae vegetation height on floodplains, with an RMSE of 0.43 m compared to the UAV-detected data; (2) Incorporating phenological and accumulated indicators into the vegetation height model can effectively improve the model accuracy by 6.8%-10.7%; (3) The vegetation height model yields an RMSE of 0.80 m compared to UAV-detected data. The outcome of vegetation height model demonstrated that the average vegetation height in 2020 decreased by 0.03-0.24 m compared to 2019 as a result of the catchment-level flood in 2020; (4) Affected by the 2020 catchment-level flood, the Poaceae that were submerged <10 days demonstrated an upward trend in their height in the next year, whereas for those submerged >10 days, an average decrease of 2.3% to 3.1% in their height was observed. Furthermore, for floodplains where the daily average submersion depth exceeded 0.95 times plant height, the negative impact of flooding gradually became severe with the increase in the ratio.