Hollow nanoparticles such as polymersomes have promising potentials in many fields. However, the design and construction of higher-order polymersomes with precisely controlled spatial compartments is still very challenging. Herein, we report a unique tetrapod polymersome that is assembled by the controlled fusion of four traditional spherical polymersomes. This original species was prepared from poly(ethylene oxide)(113)-b-poly[4-(4,4,5,5-tetra-methyl-1,3,2-dioxaborolan-2-yl) benzyl methacrylate(61)-stat-2-(diethylamino)ethyl methacrylate(23)] [PEO113-b-P(TBA(61)-statDEA(23))] in DMF/water at lower water content (C-w), where PEO acts as corona forming block. To unravel the secret behind the tetrapod polymersomes, a series of block copolymers with various comonomer types and degrees of polymerization were synthesized and self-assembled. PEO113-b-PTBA(80) self-assembles into spherical micelles in DMF/water, and the subsequent evolution into tripod and multipod micelles and finally micelle clusters was achieved by increasing C-w. This suggests that relatively rigid TBA is a "pro-fusion" component that facilitates particle-particle fusion due to its providential hydrophobicity and chain mobility. When one-fourth of TBA of PEO113-b-PTBA(80) is substituted by DEA, spherical polymersomes of PEO113-b-P(TBA(61)-stat-DEA(23)) are born in DMF/water and then fused into dipod, tripod (C-w = 95%), and finally tetrapod polymersomes (C-w = 100%) upon increasing C-w, suggesting that flexible DEA is not only a promoter for hollow pods but also an "anti-fusion" component that can compromise with pro-fusion force for its high chain mobility. The formation of either tetrapod polymersomes or micelle clusters is a matter of balance between pro-fusion and anti-fusion forces. Overall, we provide a fresh insight for the preparation of tetrapod polymersomes as well as other higher-order structures with precisely defined spatial compartments by fusion-induced particle assembly (FIPA).