During injecting high temperature and pressure steam into an oil well, it is important to economize energy and raise yield if the vapor-to-water ratio continuously from the mouth to the bottom of the oil well is known. Because the vapor and water have different refractive indexes, the vapor-to-water ratio can be deduced from the response characteristic of refractive index of two-phase flow. The developed device, which adopts sapphire as the probe, can respond to the change of vapor-to-water ratio directly, and can be used in small working space in the steam with high temperature and pressure. The response curve gained by the test in a boiler pipe is accordant with the real situation of vapor-water ratio of the steam transportation. The device also works in a steam injection oil well with depth of 800 m, temperature of 270 ℃ and pressure of 10 MPa at well mouth. The experimental result is helpful to analyze the information of steam injection well of oilfield.