Phytoplankton bloom has been observed off the coast in northern Taiwan Strait when the northeasterly monsoon wind relaxes in winter and early spring. Restratification through coastal frontal collapse may have played a significant role in the formation of the phytoplankton bloom. In this study, we conducted idealized biophysical model experiments to examine the possible formation mechanisms. First, we initialized the experiments with the observed physical–biological fields. We then seeded the initial fields with random perturbations and ran the model with different wind forcing. We ran two-dimensional (2D, cross-shore and depth) and three-dimensional (3D) experiments at different resolutions: submesoscale and mesoscale resolutions. When driven by a down-front wind, symmetric and baroclinic instabilities (SI & BCI) develop in the submesoscale-resolution 3D run. As the wind relaxes, the instabilities trigger frontal collapse as finite-amplitude meanders develop, leading to intense near-surface stratification and off-coast blooming. Symmetric instability dominates the frontal collapse in the first two days following the wind relaxation and account for more than 60% of the bloom. Subsequently, baroclinic instability becomes dominant, and by day 5, it accounts for 60% of the bloom. Restratification due to linear (i.e., Eady-type) BCI accounts for much of the phytoplankton growth in the water column. However, nonlinear mixed-layer BCI significantly contributes to the process near the surface, accounting for 30% of the total blooming in the near-surface 5 m depth of the water column.