High insertion loss filter components like tunable bandpass filters(TBPF) are commonly used to tune the sideband output wavelengths in continuous-wave(CW) all-fiber optical parametric oscillators (FOPO). Aiming at reducing the high ring-cavity loss mainly caused by the insertion-loss of the bandpass filter, a low cavity-loss tunable CW FOPO based on multimode interference(MMI) filter was proposed. Cascaded single-mode-multimode-single-mode(SMS) fiber devices were fabricated as filter devices by selecting multimode fibers with different lengths and core-sizes. And their insertion-losses at selected wavelengths were less than 1 dB, and the total losses of the FOPO ring cavity were not more than 5 dB. By applying an axial pulling force to the SMS device to adjust the transmission spectrum of the filter device, the double-sideband output wavelengths could be tunable in the range of 1 494-1 501 nm and 1 638-1 629 nm.