A novel add-drop filter based on direct coupling between micro-/nano-fiber (MNF) ring and side-polished fiber(SPF) is proposed and demonstrated. Experiment results show that this kind of filter not only can realize interconnected coupling between the MNF optical system with conventional optical system, but also can realize a function as add-drop filter between two optical systems. The maximum extinction ratio of drop port can achieve to be 7.5 dB as a drop filter, while maximum extinction of add port can be 4.8 dB as a add filter. Furthermore, the influence of different ring diameters on the coupling between the two optical systems is numerically and experimentally investigated. The conclusion of the investigation is that when the diameter of MNF is fixed at 6 μm and the diameter of MNF ring reaches to be 580 μm, coupling efficiency of the MNF ring and SPF is maximum and the influence of the coupling on the Q value and fineness of the ring reaches minimum simtaneously. This kind of add-drop filter, without assistant of extra optical device to couple light into MNF from standard optical fiber, provides a more elegant solution for interconnection of the MNF optical system and current standard optical fiber system.