A method for measuring the refractivity of saturated alcohol gas in high precision in closed vessel had been designed. This experimental apparatus used an interference method to observe the variation of interference fringes when the gas pressure changed, combining with the relationship of the number of the changing interference fringes and the refractivity of saturated alcohol gas, then the refractivity of alcohol gas under a certain pressure could be measured. The results showed that this method was accurate in deduce, simple in measurement and method, and reliable. This method could be promoted to measure the refractivity of gas. Especially, it also could be used to measure the liquid level of the liquid-storage tanks.