Microplastics (MPs) have garnered increasing research attentions due to their ubiquitous presence in the environment and consequential impacts on both ecosystems and human health. Wastewater treatment plants (WWTPs) are identified as major sources of MPs in aquatic environments, as they receive anthropogenic MPs from household and industrial sewer networks. Based on treated wastewater samples collected in October 2018 from a WWTP in Hangzhou City, China, the MP transport into the Qiantang River before heading into Hangzhou Bay, East China Sea, was investigated. To determine the abundance and characteristics of MPs, the quantities of MPs released into the environment from the WWTP in different treatment stages were estimated, and their migration was simulated using numerical modeling techniques. Results show that the abundance of MPs in the effluent of the WWTP was 1.3 pieces/L, with a daily discharge of 7.6×108 pieces of MPs into the Qiantang River. Polyether sulfone (PES), rayon, and polyethylene terephthalate (PET) were identified from MPs, being dominated by polypropylene (PP). MPs predominantly displayed fibrous and fragmented shapes. Household laundering and plastic product usage were major sources of MPs, with mechanical wear during treatment potentially contributing to the transformation from primary to secondary MPs. After 45 days of transport simulation, a high abundance zone of MPs was observed in the southern region of Hangzhou Bay, with its diffusion range being influenced by the combined effects of river runoff and tides. The spatial distribution range was significantly greater during the spring tide compared to that in the neap tide, and there were varying degrees of MP accumulation in the upper reaches of the wastewater outlet as runoff decreased. This study integrated the emissions and transport of MPs from WWTPs to estuarine rivers, thereby providing valuable scientific guidance for future regional control measures and ecological risk assessments of MPs.