The tunable optical narrow-band filter is one of the key components in an optical system, which has a wide application in many fields such as optical signal processing and optical sensing. A novel scheme for a widely tunable narrow-band filter is proposed using the micro-optofluidic technology and based on a sub-wavelength grating structure, in which a wide-range adjustable narrow-band filtering function is achieved by adjusting the refractive index of mixed liquid in the micro-fluidic channel. The optical performance is numerically investigated using the rigorous mode theory and the simulation results show that the tunable wavelength range of the designed device is over 28 nm, the filtering bandwidth is less than 0.03 nm, the sensitivity S is over 350 nm·RIU -1, and the quality factor Q is over 50000, indicating that the performance is superior. Moreover, the proposed novel device has many advantages such as simple structure, ease for fabrication and large fabrication tolerance, which is very useful in the fields of bio-sensing, chemical analysis and others.