Pomacea canaliculata (Lamarck, 1819) is a key invasive species in China, which has caused environmental catastrophes, society and ecological security issues across the country. However, our present understanding of the snail's basic biology and ecology is still insufficient, especially in the population structure and underlying driving factors. Here, we first determined the population density, key traits (body weight, shell height and width, aperture length and width) and sex ratio of P. canaliculata population across the Guijiang River Basin, the upper tributary of the Pearl River, based on three seasonal field surveys (dry, normal and wet) in 2021-2022. Then, we explored the size structure and the driving mechanisms that affected population processes. We set up 51 sampling sites in each survey. In each site, we quantitatively collected the snails using one modified sampler (3×3 m² in area) by three replicates within 100 m of each river segment, and simultaneously determined a total of 18 environmental variables with the standard methods. The collected snails were counted, photographed, measured and sexed by the morphology of the opercula and the genital organs. The size-frequency analysis was used to analyze size structure. Based on size-frequency histogram, we sorted the snails into 10 size classes according to shell height:0-20.00, 20.00-24.99, 25.00-29.99, 30.00-34.99, 35.00-39.99, 40.00-44.99, 45.00-49.99, 50.00-54.99, 55.00-59.99 and >60.00 mm. Snails < 30 mm were defined as immatures, whereas snails>30 mm were defined as matures. The local environmental variables measured showed highly longitudinal divergences across the region. A total of 1771 snails were collected, with estimated density of (2.21±2.35) ind./m². There was no significant spatio-temporal difference in density. The body weight, shell height and width, aperture length and width all showed distinctly spatio-temporal differences, with the highest values in the upstream region and during wet season. The size-frequency distribution was distinctly skewed towards the large (adolescent and postadolescent), mature individuals (30-50 mm, 54.2% of total individuals), indicating a stable population. The sex ratio (male:female) was 0.36±0.40, but showed no spatio-temporal heterogeneity. Canonical correspondence analysis indicated that local factors (pH, altitude, dissolved oxygen, and salinity) and large-scale spatial factors (PCNM2, PCNM5, PCNM8 and PCNM9) were the key regulators affecting the size structure. Variance partitioning indicated that both the environmental and spatial factors significantly affected the size structure, with environmental filtering generally more important. The study can provide theoretical reference for establishing effective prevention and control strategies of P. canaliculata.