The refractive index profile of optical fiber preform can be measured by beam scanning method. Due to the requirements of the refractometer for the length, diameter and shape of preforms, the refractive index profile of special-shaped preforms cannot be measured directly. Thus the sleeve-assistant method is produced. A quarts sleeve is needed to expand the measuring range of the refractometer. The refractive index profile of the same round preform is measured directly by refractometer and by sleeve-assistant method respectively. And the results obtained are consistent, confirming the accuracy of the sleeve-assistant method. The refractive index profiles of the neck-down region of the preform measured confirm that the sleeve-assistant method can improve the test ability of refractometer.