The characteristics of periphytic diatoms serve as effective indicators of changes in aquatic environments. However， research on the characteristics of periphytic diatom communities and their associated environmental factors in large urban river networks remains limited. Our study focuses on the urban river network of Shanghai City， selecting 16 medium and small rivers to investigate periphytic diatoms by using artificial substrates. The Trophic State Index （TSI） method was employed to comprehensively assess water quality. The characteristics of the periphytic diatoms communities were analyzed， and the threshold response of these communities to soluble nitrogen and phosphorus nutrients was quantified.Our results indicate that， with the exception of two sites in Shujiawan River， Baoshan District， most locations are eutrophic. The main genera with a relative abundance exceeding 10% include Navicula， Ulnaria， Nitzschia， Melosira， and Gomphonema. A total of 28 species with relative abundances greater than 1% were identified， with Melosira varians being the most dominant. Redundancy analysis （RDA） revealed that the primary factors influencing differences in periphytic diatom communities are dissolved total nitrogen （DTN）， dissolved total phosphorus （DTP）， ammonia nitrogen （NH₄⁺-N）， silica （SiO₂）， and chemical oxygen demand （COD_Mn）. Furthermore， threshold indicator taxa analysis （TITAN） indicated that the periphytic diatom communities undergo significant changes when the thresholds of DTN and DTP are reached.The positive response threshold for DTN is 2.26 mg/L， while the thresholds for DTP are 0.12 mg/L （positive response） and 0.16 mg/L （negative response）. Fourteen species exhibited a positive response to DTN， with Gomphonema angustatum showing the highest threshold at 3.30 mg/L. Fifteen indicator species responded positively to DTP， with Gomphonema acuminatum and Gomphonema gracile both displaying a threshold of 0.24 mg/L. Only two species showed negative responses: Gomphonella olivacea （threshold 0.14 mg/L） and Navicula cryptotenella （threshold 0.17 mg/L）， while Nitzschia palea was the most frequently observed species， occurring 31 times.The results demonstrate that key water chemical parameters in small and medium rivers of Shanghai significantly influence the structure of diatom communities. Species such as Gomphonema gracile can serve as bioindicators for dissolved total nitrogen （DTN） and dissolved total phosphorus （DTP） levels in water bodies. This study provides foundational data and technical support for ecological monitoring and scientific management of Shanghai's river network.