According to the optical refraction characteristic of transparent medium, the measurement error of internal standard method is analyzed, and then an improved method of liquid refractive index measurement using laser speckle interferometry is proposed. This paper designs a double-groove structure composed of two identical rectangular grooves with one of the grooves filled with the measured liquid, and then an image speckle optical measurement system has been built, while the double-groove structure is placed between the imaging lens and CCD camera. First, adjusting the two grooves to be parallel and perpendicular to the optical axis, one speckle pattern is collected by CCD camera. Then adjusting the included angle of two grooves to make them symmetrically deflect to a small angle, another speckle pattern is collected. After that, a speckle in-plane displacement before and after the change is obtained by processing the above two speckle patterns using the digital speckle correlation method, and finally the refractive index of transparent liquid can be calculated theoretically. According to the above method, the refractive indices of water and ethanol are measured, and the results show that this improved method has the advantages of small error and high accuracy, compared with internal standard method.