An on-chip micro-droplet optical filter based on the evanescent wave coupling by fiber taper is presented in this paper. The optical filter consists of a T-junction for micro-droplets generation and an integrated optical fiber taper for light input and output. The light is coupled into the droplet resonator when it has the same wavelengths as the Whispering Gallery Modes (WGMs) of the droplet. The coupling efficiency can be adjusted by tuning the distance between the droplet and the fiber taper using micro fluidic flow. Compared with the solid sphere or ring optical filter using fiber taper coupling, the diameter and refractive index of liquid droplet can be easily tuned, which means the resonant modes can also be tuned. The diameter of the droplet is controlled by the flow rate ratio at the T-junction. The micro-droplet optical filter also has advantages as wide wavelength tuning range, easy fabrication process, simple manipulation, and low cost, which promise for many applications in optical analysis on micro-optical-fluidic chip.