Based on the space curve meshing theory, a variable-ratio line gear mechanism which has the deformable teeth was proposed. The basic design theory of the variable-ratio line gear mechanism was studied and the transmission ratio can be changed by deforming the line gear teeth. The change rule of the transmission ratio for the variable-ratio line gear mechanism was analyzed, it was found that the variable-ratio line gear mechanism can achieve multistage transmission. Then, a design case of variable-ratio line gear mechanism was developed and a homemade test rig was made. Experiments were carried out on the design case to verify the basic design theory of the variable-ratio line gear mechanism. It was shown that the variable-ratio line gear mechanism can realize stable transmission at several ratios only depending on two gears. The variable-ratio line gear mechanism was a new gear mechanism, which can be used in applications with light load.