The major data of "Southwestward intermittently intrusion of Changjiang Diluted Water in Hangzhou Bay against southwesterly monsoon" includes following parts: 1. Observed difference of dissolved inorganic nitrogen (DIN) concentration (color shadings) between the summer and winter cruises in Hangzhou Bay. 2. Comparison of observed and simulated particle trajectories in Hangzhou Bay (2022). 3. Mean surface and bottom flow patterns in Hangzhou Bay during summer and winter, averaged from 2019 to 2022. 4. The average distribution of surface and bottom salinity in Hangzhou Bay from 2019 to 2022 for summer and winter. 5. The standard deviations of current and salinity in Hangzhou Bay during summer. 6. Daily salinity variation along transect across P1. 7. Daily salinity, CD, wind stress at P1 and P2. 8. Distributions of Lagrangian residual currents, Eulerian residual currents, and Stokes drifts in Hangzhou Bay during mean tide, neap tide, and spring tide, respectively. 9. Magnitude ratio of the tidal residual current to the mean flow in summer (average over 2019–2022). 10. Spatial distribution of CDW in Hangzhou Bay during summer and winter, respectively. 11. Latitude and longitude coordinates of points P1 and P2.