A series of incomplete-cage polyhedral oligomeric silsesquioxanes (DVPOSSs) bearing vinyl groups (-CH=CH2) and phosphorus-containing groups (DOPO) have been synthesized through a hydrolytic condensation reaction. FTIR, NMR, and MALDI-TOF MS have been used to identify the structures of the DVPOSSs, which include T9, T10, and T11 cage structures. Polycarbonate (PC)/DVPOSSs composites can be achieved through simple twin-screw extrusion. The addition of DVPOSSs improves the haze of PC/DVPOSSs composites, while maintaining the high transmittance of PC control. The thermal properties and fire behavior of the PC/DVPOSSs composites have been investigated by TGA, DSC, cone calorimetry, and LOI and UL-94 tests. PC/DVPOSSs-2 and PC/DVPOSSs-4 attained a V-0 rating in 3.2-mm and 1.6-mm samples. The pyrolysis and flame-retardant mechanisms of the PC/DVPOSSs composites have been assessed on the basis of TGA-FTIR, cone calorimetry, and char layer morphology results. The main flame-retardant mechanism is that DVPOSSs induce early decomposition of the PC matrix and increase char formation, rapidly extinguishing the flame and causing an obvious reduction in heat release rate.