Gold nanoparticles confined into the walls of periodic mesoporous organosilicas (PMOs) with controllable morphology have been successfully fabricated through a one-pot method by using different CO2 pressures. The synthesis can be easily conducted in a mixed aqueous solution by using HAuCl4 as gold source and bis[3-(triethoxysilyl)propyl] tetrasulfide and tetramethoxysilane as the organosilica precursor. P123 and compressed CO, served as the template and catalytic/regulative agent, respectively. Transmission electron microscopy, N-2 adsorption, and X-ray diffraction were employed to characterize the structure of the obtained composite materials. To further investigate the formation mechanism, a series of ordered PMOs with one-dimensional nanotube, two-dimensional hexagonal, vesicle-like, and cellular foam structures were obtained by using different CO, pressures without the gold source. The mechanism for mesostructure evolution of PMOs with different CO, pressures was proposed and discussed in detail. The catalytic performance of Au-based PMOs was evaluated for the reduction of 4-nitrophenol (4-NP). These obtained composites with different mesostructures not only exhibit excellent catalytic activity, high conversion rate, and remarkable thermal stability, but they also exhibit morphology-dependent reaction properties in the reduction of 4-NP. The possible reaction pathway of the reactants to embedded Au active sites was proposed and schemed.