Micro-tube biosensor is established with hollow optical fiber and its whispering gallery mode (WGM) is analyzed theoretically. The principle of bio-sensing based on the change of refractive index is presented. With the prism of ZF13, WGM of micro-tube is successfully observed by evanescent coupling. The mixture of water and ethanol is used for experimental research on sensitivity with the change of refractive index of micro-tube biosensor. At the resonance wavelength of 1555.0 nm, 2.2 nm/RIU sensitivity is obtained with 45° incident angle and 21.2 nm/RIU is obtained with 36° incident angle.