Sunflower is one of the main oil crops in China. Its straw is a natural cellulose material, which has the advantages of being green, non-toxic and low cost, and it is an ideal material for synthesising biomass carbon quantum dots. In recent years, due to the abuse of copper-containing pesticides and chemical fertilizers, many copper-containing pollutants have been discharged, resulting in the copper content in farmland soil and water environment being much higher than the environmental background value. Therefore, it is urgent to develop a Cu2+ detection method with good selectivity, high sensitivity and environmental friendly. Carbon quantum dots (CDs) are quasi-spherical fluorescent carbon nanomaterials with particle sizes less than 10 nm. Its surface contains abundant polar functional groups and has been widely studied for its good water solubility. Compared with traditional semiconductor quantum dots (CdSe, CdTe), CDs have extensive synthetic materials and good biocompatibility advantages. They are mainly used in biological imaging, photo catalysis, photoelectric conversion, sensor detection and other fields. However, most of the precursors of carbonized CDs synthesis are expensive chemicals, which have the disadvantages of a complex synthesis processes and environmental pollution, limiting the large-scale production and application of CDs. Therefore, developing an eco-friendly, simple and inexpensive CDs synthesis method is of great significance. In this study, waste sunflower straw was used as a carbon source, and a simple hydrothermal method was used to synthesize biomass carbon quantum dots (S-CDs) as fluorescent probes to detect Cu2+. Through a series of optical properties analysis and characterization of S-CDs, the surface functional groups of S-CDs mainly include O-H, N-H andCN, among which abundant O-H can provide colloid stability, effectively control the morphology and particle size distribution of nanoparticles, thus improving the quantum yield (QYs) of S-CDs. ex=317 nm, em=456 nm, S-CDs have excellent optical performance and good optical stability in the pH range of 2.0~12.0, less affected by high salinity environment, QYs is about 8.42%, emitting blue fluorescence under UV analyzer irradiation at 365 nm. In addition, the fluorescence quenching effect of S-CDs induced by Cu2+ was further studied using the synthesized S-CDs as a fluorescence probes. The results showed that the prepared S-CDs were sensitive to Cu2+ in the concentration range of 0～10 μmol·L-1 with a good linear relationship (R2=0.971 4), and the detection limit (LOD) was as low as 167 nmol·L-1. In practice, the detection recoveries of Cu2+ in lake water are 96.18%～109.22%. In this study, a Cu2+ detection method based on fluorescent carbon quantum dots was introduced based on resource utilization of straw waste.