The refractive index of seawater is one of the important optical properties of the air-sea interface, which is of great significance to the study of ocean optics, ocean dynamics, and air-sea interaction. The refractive index of seawater refers to the ratio of the propagation speed of light waves in a vacuum to that in seawater. In this paper, the refractive index of seawater is measured quickly and accurately by a spectrophotometer combined with the double triangular prism method. By comparing with the measured data of the Abbe refractometer, the measured data of this experimental method are accurate and reliable, which can be extended to the rapid measurement of the refractive index of other liquids by simple instruments. The double-prism method for rapid measurement of seawater refractive index is to use the existing simple equipment to measure the liquid refractive index. It provides accurate optical parameters for investigating the optical properties of the ocean or the propagation law of light in the sea and provides accurate optical parameters for studying marine optics.