In this paper we propose a magnetically tunable plasmons resonator based on a graphene-coated nanowire. Due to the magneto-optical effect under an external magnetic field, the circumferential propagation of graphene plasmons on a magneto-optical nanowire becomes non-reciprocal with the modal indices depend on plasmons traveling directions (clockwise or anti-clockwise). When coupled with a graphene sheet waveguide, the two components form a graphene plasmons filter for which the shift direction of transmittance spectrum is determined by the direction of input plasmons. The resonant wavelengths of resonator are obtained through resonant cavity theory and verified by numerical solutions. Furthermore, the non-reciprocal transmittance enables such structures to achieve the function of a plasmons isolator where the isolation could be tuned by the amplitude of an external magnetic field and the enabled plasmons propagation direction could be switched by reversing the direction of external magnetic field. Under proper structural parameters and magnetic field, an isolation ratio over 25 dB is obtained. The proposed magnetically tunable plasmons resonator may provide new inspiration to graphene plasmonics devices.