Biological small molecules, also known as monomeric compounds with relatively low molecular weight found in organisms, encompass a wide array of substances in plants, such as ions, plant hormones and metabolites. Studying the dynamic fluctuations of these small molecules in plants is crucial for analyzing their corresponding physiological functions, regulatory networks, and enhancing the precision of botanical research. Genetically encoded fluorescent biosensors/probes utilizing Förster resonance energy transfer (FRET) technology serves as a valuable tool for real-time monitoring of these small molecules within living organisms. These FRET biosensors/probes allow for the non-invasive visualization of specific small molecule concentrations, providing detailed information at a high resolution. Because of these unique advantages, this technique has been extensively applied in various research fields, including plant physiology, developmental biology, and environmental science. This review provides a comprehensive overview of FRET sensors/probes utilized in plant research in recent years, outlines the key design concepts, and highlights their applications and advances in detecting ions, plant hormones, and metabolites. Furthermore, this review demonstrates practical technological tools and potential research directions for elucidating the functions of small biomolecules in plants.