Abstract: Objective To analyze the infection characteristics of human respiratory syncytial virus (HRSV) in patients with acute lower respiratory tract infection (ALRTI) in Shanghai children's hospitals, so as to provide evidence-based support for optimizing the prevention and control strategies and clinical diagnosis and treatment of respiratory tract infections in children in this region. Methods A retrospective analysis was performed on a total of 29260 children hospitalized for ALRTI in Shanghai Children's Hospital from January 2021 to December 2024. HRSV and 12 other common respiratory pathogens were detected with multiple polymerase chain reaction (PCR) and capillary electrophoresis. Demographic and clinical data were collected for statistical analyses. A total of 2412 cases of children with positive HRSV were divided into the severe group and the non-severe group. The Mann - Whitney U tests and the chi - square (χ²) tests were employed to compare the clinical characteristics between the two groups. Additionally, the related influencing factors of severe HRSV infection were explored. Results The total positive rate of HRSV from 2021 to 2024 was 8.24% (2412/29260), and there were significant differences in the HRSV positive rates across different years (χ² = 389.42, P < 0.001). The HRSV positive rate was the highest in 2021 (14.76%), with a high prevalence throughout the year. In 2022, when non-pharmaceutical intervention measures (NPIs) were implemented, the HRSV positive rate was the lowest (4.93%), with a winter-dominant epidemic pattern. In 2023, after the NPIs were lifted, the HRSV positive rate showed a slight rebound (8.14%), and the epidemic pattern presented a double-peak pattern. In 2024, the HRSV detection rate slightly decreased compared to that in 2023 (6.29%), with a winter and spring-dominant epidemic pattern. Among the hospitalized children with ALRTI, the HRSV positive rate in males (8.85%) was higher than that in females (7.51%), and the difference was statistically significant (χ²= 17.33, P< 0.001). Among children aged 3 years old and below, the proportion of HRSV infection was 82.26% (1984/2412); as age increased, the infection rate of HRSV showed a gradually decreasing trend (P< 0.001). Among the 2412 children with HRSV infection, the proportion of severe cases was 22.31% (538/2412), while the non-severe cases accounted for 77.69% (1874/2412). Compared with non-severe HRSV-infected children, severe HRSV-infected children had more frequent high fever, longer duration of wheezing, and higher rates of underlying diseases and mixed mycoplasma infection (P < 0.01). Conclusions The prevalence intensity of HRSV varied in each year from 2021 to 2024. After the removal of NPIs in 2023, there was a slight rebound and a double-peak epidemic pattern. HRSV was a common pathogen for children hospitalized with ARLTI, and children under 3 years old constituted the highest proportion for infection. Compared with non-severe HRSV-infected children, those with severe HRSV infections were more prone to presenting with high fever and a longer duration of wheezing. Children with positive HRSV who had underlying diseases and mixed mycoplasma pneumonia were more likely to develop severe conditions.