The application discloses a variable-index viscous damper. First damping channels and second damping channels are formed in a barrel body of a cylinder barrel, and circulating holes which enable the first damping channels and the second damping channels to communicate with each other are further formed, and the output force of the damper is adjusted through the positions of the different circulating holes. The application further discloses an another variable-index viscous damper, first damping channels, third damping channels and fourth damping channels are formed in a barrel body of a cylinder barrel, circulating holes which enable the first damping channels, the third damping channels and the fourth damping channels to communicate with one another are formed, and the output force of the damper is adjusted through the positions of the different circulating holes. When a piston of the damper moves, the damper can generate the output force with different indexes to adapt to irregular change of external force so that the protective function of the viscous damper to a building structure can be enhanced.