The invention discloses an SPPs mode converter and conversion method based on an asymmetric nanoscale groove structure. The surface plasmon polaritons (SPPs) mode converter comprises a metal film, a main nanoscale groove is formed in the surface of the metal film, an auxiliary nanoscale groove is formed in the one side of the bottom of the main nanoscale groove to form the asymmetric nanoscale groove structure, interconversion coefficients of a first-order waveguide mode and a second-order waveguide mode in the main nanoscale groove are controlled by controlling the depth of the auxiliary nanoscale groove of the resonant structure, and therefore efficient and controllable conversion of surface plasmon polaritons of different modes is achieved; the efficiency of mode conversion can reach up to 90%. The total electromagnetic field distribution is more convenient to control through interference between the first-order waveguide mode and the second-order waveguide mode, meanwhile, the SPPs mode converter has the super-small size of a few hundreds of nanometers, high integration is facilitated, and therefore the SPPs mode converter is widely applied in an SPPs photonic circuit of the ultra-high integration level.