In this paper, the refractive index of GdTaO4 crystal is measured by the auto-collimation method. GdTaO4 crystal is processed into three rectangular prisms, their planes with longer right angle side are planes a, b and c of the crystal, respectively, and their normal directions are the directions of crystal plane axes a, b and c, respectively. with a side by angle . Plane enclosed by hypotenuse and the longer right angle side is subjected to fine polishing, while the surface plating for the latter is subjected to Al reflectance coating, so that the light is incident along the hypotenuse plane with minimum angle of deviation (), reflects on the plane with longer right angle and returns along the original path. The rectangular prisms processed by GdTaO4 crystal are placed on the platform of 32 J goniometer with an accuracy of arc seconds. The 473, 532 nm YAG double frequency laser, 633 nm He-Ne laser and 1064 nm YAG laser with stable light intensity are used as a measuring light source, light will refract into the light perpendicular to the longer right angle side when the laser of measuring light source shoots towards the bevel of a prism with a minimum angle of deviation (). The refractive indexes nx, ny, and nz of a crystallographic axis directions can be measured by , and the relationship between refractive index ellipsoid section and prism refraction of light. The constants Ai, Bi, Ci, and Di (i=x, y, z) are given in Sellmeier's equation ni2 =Ai+Bi/(2-Ci)-Di2, and the values of angle Vz included between light axis and refractive index at wavelengths of 473, 532, 632.8 and 1064 nm are calculated to be 22.5, 22.5, 21.9 and 22.0, respectively. It is proved that GdTaO4 crystal is optically positive biaxial crystal.