A layer of gold film is deposited on glass substrate by magnetic sputtering and different concentrations of sodium chloride solutions are drop on gold film as test samples. Refractive of sodium chloride solution is obtained by analyzing the coupling of light and surface plasmon produced on the interface of gold film and liquid，the relative position of dark ring in the back focal plane，and the theory of surface plasmon. Experimental error is calculated and the method of improving the measurement precision is proposed. This experiment combined micro and nano fabrication with the back focal plane imaging process，which has been developed a new way to measure refractive index of liquids and then has important applications in material analysis and sensor.