In engineering construction, foundation pit drainage is a necessary measure to ensure the safety of the foundation pit. The water discharged into the urban inland river impacts the ecological safety of the inland rivers and downstream water. In this study, the drainage water from the foundation pit (W1), 100 m upstream of the drainage outlet (W2), the drainage outlet (W3), 50 m downstream of the drainage outlet (W4), 100 m downstream of the drainage outlet (W5) and 200 m downstream of the drainage outlet (W6) were collected in the process of construction in Eurasian Window Park reach of Hejia river, Harbin city. Three-dimensional fluorescence spectrum - parallel factor analysis method was used to determine the fluorescence spectrum characteristics of dissolved organic matter (DOM), analyze the composition and source of DOM, and explore the impact of foundation pit drainage on the urban inland water environment. The results showed that the humification index (HIX) of inland water was in the range of 0.337~0.381, and the humification degree was low. There was no significant difference in the HIX of W1, W3~W6, which was significantly lower than W2, indicating that drainage further reduced the humification degree of inland water. The fluorescence index (FI₃₇₀) was in the range of 2.330~2.900, and the biological index (BIX) was 0.897~1.140. The FI₃₇₀ and BIX of W1 and W2 were significantly higher than those of W3~W6.Both of them had strong autochthonous characteristics, which indicated that the drainage reduced the autochthonous characteristics of the downstream water. The water DOM identified two types of four organic components: visible fulvic-like component (C1), tryptophan-like component (C2), ultraviolet fulvic-like component (C3) and tyrosine-like component (C4), namely fulvic-like substance (C1, C3) and protein-like substance (C2, C4). There was a negative correlation between the two types of substance. The correlation between FI₃₇₀ and four organic components was very significant, indicating that DOM's composition was simple. W2 had a relatively high DOM concentration, while the DOM concentration downstream of the outlet was low and stable. Protein-like substances occupied a relatively high proportion in the upstream water. In W4, there was no significant difference in the relative proportion of the four organic components. In W5 and W6, the relative proportion of fulvic-like substance increased later, which also indicated that the drainage of the foundation pit led to the decrease in the autochthonous characteristics of inland water. Except for the increase in pH value, the contents of dissolved oxygen (DO), total N, total P and other physicochemical indexes of downstream water samples decreased. pH value was positively correlated with fulvic-like substance and negatively correlated with protein-like substance, while DO, chemical oxygen demand and water nutrient indexes were contrary. The correlation between water DOM components and physicochemical indexes was different, directly or indirectly affecting the DOM composition. Therefore, in engineering construction, the drainage of the foundation pit reduced the DOM concentration and changed the DOM composition of urban inland water.