Complex emulsions including double emulsions and high-internal-phase emulsions (HIPEs) are wonderful templates for producing porous polymeric materials. Yet, surfactants and multiple emulsifications are generally needed. In this work, surfactant-free complex emulsions are successfully prepared using a CO2-responsive block copolymer through one-step emulsification. Phase inversion from HIPEs to double emulsions happens in one system upon the change in polymer amphiphilicity as a result of CO2 triggering. The one-step emulsification method offers great convenience for converting the block copolymer into porous 3D scaffolds and particles. Moreover, CO2 triggering is erasable so that the polymer can be repeatedly used for controllable complex emulsions as well as porous materials.