This study utilizes the quasi-Lagrangian azimuthal momentum equation (i.e., budget calculation) and 1.667-km-resolution numerical simulation data to study the intensity and structural changes in Hurricane Sandy’s extratropical transition. The results indicate that after the onset of extratropical transition, Sandy maintains an eyewall-like convection and warm core in the core region and has a frontal structure in the outer region. In the outer region, baroclinicity-driven frontal convection induces extensive planetary boundary layer (PBL) inflow, causing an inward advection of absolute angular momentum (AAM) per unit radius, which generates outer local wind maxima and expands Sandy’s outer wind field through a spinup process. Moreover, because the outer tangential wind velocity accelerates in a frontal convection, local wind maxima associated with fronts can expand to the outer sides of frontal regions. Frontal convection increases AAM in the outer region, providing the precondition for reintensification; however, the front itself cannot cause Sandy’s reintensification. The eyewall-like convection in the core region still plays an important role in Sandy’s reintensification. When the baroclinic zone, where a strong horizontal temperature gradient exists, approaches the core region, the eyewall-like convection is enhanced because the warm, moist air of the core region is lifted by the cold, dry air associated with the approaching baroclinic zone. Consequently, owing to the enhancement of eyewall-like convection, the PBL inflow, which extends from the outer region to the core region, develops. This inflow increases the inward transportation of the outer frontal region’s high-AAM air, thus leading to spinning up the core region’s wind and reintensification.