The refractive indices of nonlinear optical crystal YAl3(BO3)4 (YAB) for nine wavelengths of 0.194, 0.254, 0.404, 1.53, 2.325 μm and so on under the temperature range from 40 ℃ to 160 ℃ are measured by the prism minimum deviation method．The Sellmeier equations at different temperatures and the relationship between the thermal coefficients of refractive index and the wavelength have been obtained by the least square method. The calculated acceptance temperature for type I phase-matching of YAB crystal for 266 nm ultraviolet light generation is 6.9 ℃·cm based on the above equations. Both the experimental and theoretical values show that YAB crystal may be an ideal nonlinear optical crystal for 266 nm harmonic generation with high power and high stability because of its good thermal stability.