To achieve single-mode optical microfluidic laser, we prepare Fabry-Perot(F-P) optical microcavity with high quality factor and use the organic dye Rhodamine 6G (R6G) dissolved in liquid as gain medium. The full width at half maximum of laser output is 0.260 nm. In the following experiments, the generation of single-mode lasers in different mixed solutions (absolute ethanol and deionized water) with water molar fractions of 1.09%，5.98%，11.91%，20.42%，30.75%，45.27%，51.89% is achieved, respectively. It is found that the center wavelength of the single-mode laser moves toward long wavelength as water contents increasing. However, when the water molar fraction exceeds 45.27%, the center wavelength of the single-mode laser starts to move toward the short wavelength. Measurement of mixed solution refractive index is realized by single-mode laser wavelength movement. It is found that with the increase of water content, the measurement and analysis of the refractive index of the mixed solution show that the minimum measurable difference of refraction indexes is 6.31×10-4 and the measurement sensitivity is 411 nm/RIU, in which RIU is refractive index unit. The measured refractive index results and errors are analyzed.