KTiOPO₄(KTP) and KTiOAsO₄(KTA) crystals are new media to generate terahertz (THz) waves.In this paper,a method for measuring the refractive index of KTP and KTA crystals at 3－6THz is demonstrated.The Stokes light with wide spectrum was obtaine d,using an Nd:YAG 1064nm laser as pump source and a KTP (KTA) crystal as the nonlinear medium in the tera hertz-wave parametric generation.The 1064nm laser has a pulse width of 5ns and a pulse repetition rate of 1Hz.The crystals with dimensions of 5mm×5mm×20mm are cut at an x axis.Usi ng a collimator,the diameter of 1064nm laser is converted to 1.5mm.The Stokes wavelengths and the corresponding angle between pump and S tokes waves are measured. The refractive index of KTP crystal is calculated to be 2.81－5.05at 3.07－6.12THz,and the refractive index of KTA crystal is calculated to be 3.26~ 4.68at 3.71－6.19THz.