We demonstrate a new mode-locking method: multimode interference mode-locking. This method is simple and convenient in construction. It is only necessary to fuse two short pieces of graded-index multimode fiber in a single-mode fiber laser, which uses the mode interference effect of single-mode multimode single-mode (SMS) structure to achieve saturable absorption mechanism. In order to realize the mode-locking of the SMS structure, it is necessary to precisely control the length of multimode fiber. We propose to coil the SMS structure into the polarization controller. By theoretically deriving the polarization controller to adjust the phase of transmission light in a multimode fiber, the saturable absorption effect can be achieved. Under the 263 mV pump power, a stable 24.83 MHz repetition frequency fundamental frequency mode-locked pulse output was realized, where the pulse interval was 40.12 ns, the signal-to-noise ratio was 50.8 dB, and the center wavelength was 1881.7 nm. The conversion between soliton molecules and traditional soliton can be realized by adjusting the polarization controller and pump power. Under the pump threshold of 410 mW, a stable 25 MHz repetition frequency soliton molecular mode-locked pulse output was realized, where the pulse interval was 40.3 ns, the signal-to-noise ratio was 54.4 dB, and the center wavelength was 1887.60 nm.