A fiber laser micro-nano sensor based on colloidal crystal structure is proposed in this paper. The fiber laser has stable frequency and narrow linewidth. It is realized by using an unpumped erbium-doped fiber (EDF) as the saturable absorber. The saturable absorber possesses the shape of taper. The laser threshold can be effectively reduced by the tapered saturable absorber. The tapered fiber coated with colloidal crystal as sensing unit is studied. The concentration of ethanol can be obtained from the detection of the output laser wavelength. It can be extensively used in chemical, medical and biological detections.