Tianmu Lake is the important water resource in the Jiangsu Province, which is composed by Daxi and Shahe Reservoirs with abundant fishery resource. According to the spatio-temporal variation characteristics of main fishes' food web in Daxi and Shahe Reservoirs, the stable isotope ratio data of species were collected from two reservoirs from July 2017 to April 2018, and the nutrient relationship was analyzed for key species at first time. The results showed that 32 fish species were captured from the surveys, of which 28 fish species were sampled from Daxi Reservoir, while 29 fish species were sampled from Shahe Reservoir. The fish structure communities in two reservoirs were similar, dominated by the sedentary fishes and Cyprinidae. The spatio-temporal differences of δ¹³C value and δ¹⁵N value were analyzed by the paired samples t-test method. The results indicated that there were significant differences of δ¹³C value between the two reservoirs during the same season. There were no significant differences between each season in Daxi Reservoir but opposite in Shahe Reservoir. It means that two reservoirs had various composition of food sources, and that composition of basal resources of fishes in Daxi Reservoir may be more stable than that in Shahe Reservoir. In addition, there were no obvious variations in spatio-temporal of δ¹⁵N value, which may be related to the pollution of agricultural and human activities around the reservoir. The index calculation results showed that range of δ¹³C (CR) and range of δ¹⁵N (NR) in Shahe Reservoir were higher than those in Daxi Reservoir, indicating that the food sources were more abundant and the food chains were longer in Shahe Reservoir. Also, the area of convex hull(TA) and standard ellipse area corrected for small sample size (SEAc) value were higher than Daxi Reservoir, meaning that the trophic levels were more complicated and the core niche space was narrower. Moreover, the mean distance to centroid(CD), mean nearest-neighbor distance(MNND) and standard deviation of nearest-neighbor distance(SDNND) values were also higher in Shahe Reservoir, showing that the overall degree of trophic diversity was higher but lower evenness nutrient niches in it, while higher redundancy in Daxi Reservoir. In addition, seasonal difference analysis results showed that the six index values were higher in spring than other seasons, meaning that the food web structures in spring were more complex. Comparing with Daxi Reservoir, the investigation showed that Shahe Reservoir had lower stability and less anti-interference ability, which may be correlated with the extensive distribution of tea gardens, tourist and residential area around it. Therefore, based on the quantitative index analysis results above, the study could provide adequate theoretical support for the management and restoration of aquatic ecosystem of Daxi Reservoir and Shahe Reservoir.