A watershed's economic development can significantly impact the water quality of drinking water reservoirs, with one important aspect being the exacerbation of dissolved organic matter (DOM) pollution through river inputs. However, there is a lack of systematic studies on the spatiotemporal variability of DOM composition in drinking water reservoirs due to river inputs. Based on synchronous field sampling of the Shahe Reservoir and its inflowing rivers during different seasons, this study explored the spatiotemporal variation characteristics of DOM composition and the influence of river inputs using methods such as parallel factor analysis of three-dimensional fluorescence spectra of colored dissolved organic matter (CDOM). We obtained the following results. ①Reservoir DOM contained four components: humic substance C1, protein-like tyrosine C2, protein-like tryptophan C3, and terrestrial humic substance C4, with C2 accounting for over 50% of the proportion at most sampling points. ②The composition of reservoir DOM exhibited significant seasonal variation, with humic substance DOM components being lowest in spring; for the main component C2, the content proportions in spring, summer, autumn, and winter were 46.66%, 34.58%, 54.74%, and 56.00%, respectively. ③River inputs had a decisive impact on the spatial distributions of reservoir DOM, with the contents of components other than tyrosine, the main source of which was domestic sewage, being higher in the river mouth area. Although the DOM input from rivers to reservoirs was mainly terrestrial humic substance C1 (with proportions of 28.80%, 30.51%, 27.11%, and 22.19% in spring, summer, autumn, and winter, respectively), the degree of humification of DOM in Shahe Reservoir was low, indicating that autochthonous sources dominated the DOM in the reservoir. Autochthonous DOM in the reservoir was mainly related to algal proliferation, and the component of protein-like tryptophan C3 was linearly significantly positively correlated with chlorophyll-a content (R2=0.51, p<0.01). This study is of great significance for improving the water quality of drinking water reservoirs, reducing organic pollution, and ensuring the safety of residents drinking water.