Abstract: Abstract: Frequent outbreaks of cyanobacteria bloom in Dianchi Lake pose a potential threat to ecosystem and human health.Aiming to realize the rapid monitoring of algal toxins in Dianchi Lake, and clarify its spatiotemporal distribution characteristics and driving factors,, this study developed an on-line solid phase extraction method for the simultaneous determination of 8 kinds of algal toxins in surface water using ultra-high performance liquid chromatography mass spectrometry (UPLC-MS/MS) through the investigation and sampling of 37 spots in Dianchi Lake. The method is simple and fast, with high sensitivity and accuracy. This method was used to analyze the spatiotemporal distribution characteristics of algal toxins in Dianchi Lake and their correlation with environmental factors. The results showed that the temporal distribution of the concentration of algal toxins in Dianchi Lake was summer (141.5475-896.8037 ng/L) > spring (51.2630-1245.2583 ng/L) > autumn (28.4733-409.1775 ng/L) > winter (16.2716-122.8470 ng/L); In space, the central area of Dianchi Lake (Guanyin Shanxi - Guanyin Mountain) has detected high concentrations of MCs throughout the year. At the same time, the overall concentrations of the outer sea and the northern part of Caohai have increased significantly in spring and summer. In terms of composition of algal toxins, MC-LR is the main toxin species in Dianchi Lake, followed by MC-LW, MC-RR and MC-WR, and the proportion of MC-LR and MC-LW is significantly affected by temperature. There is a significant negative correlation between the concentration of algal toxins and the biomass of algae, which may be related to the special hydraulic conditions and flow field of Dianchi Lake; There is a significant positive correlation between algal toxins and chlorophyll concentration Chl-a, a significant positive correlation with NH₄ -N, TP and permanganate index, a significant negative correlation with TN, a very significant positive correlation with BOD₅, and a significant positive correlation with transparency. The above results show that light intensity (transparency) and nutrient salts (nitrogen, phosphorus, organic matter) are important factors to control the production of MCs in Dianchi Lake, providing a scientific basis for the prediction, early warning, and ecological risk prevention and control of MCs in Dianchi Lake.