Self‐assembly is an appealing strategy for preparing nanospheres with different interiors, which are essential for their applications. Although many assembly strategies have been proposed, controlling the assembly processes from kinetic aspects is a big challenge. Here, by employing the different reaction kinetics of the assembly precursors, a sequential assembly strategy is proposed to tailor the interior structure of porous carbon spheres. Through changing the feeding interval of resin and silica precursors from 0 to 60 min, their nucleation order can be controlled in the assembly process to prepare porous carbon spheres (≈450 nm in size) with tunable type (i.e., hollow or solid) and size (from less than 100 nm to around 230 nm) of interiors. The hollow spheres exhibit over three times the catalytic activity of the core–shell counterparts for activating peroxymonosulfate to remove organic water contaminants, and the activity can be further improved by decreasing the cavity size. These results show the great significance of the sequential assembly strategy for interior engineering of nanospheres. This work opens up a new approach for rational design and synthesis of interior‐structured nanospheres. A sequential assembly approach is developed by simply changing the feeding interval of the carbon and silica precursors for preparing the porous carbon spheres with tunable type (i.e., hollow or solid) and size of interiors. The interior engineering of the carbon spheres significantly enhances their performance in catalytic water decontamination through peroxymonosulfate activation.