A planar optical waveguide is formed in monoclinic double rare-earth-tungstate laser crystal Yb:KLu(WO4)2 by 6.0 MeV oxygen ion implantation with a dose of 2 × 1015 ions/cm2 at room temperature. Subsequently, annealing at 300℃ for an hour in air is performed on the sample to decrease colour centres to improve the thermal stability of the waveguide. The refractive index profiles of the waveguide are reconstructed by an effective refractive index method. Dark modes of the waveguide are observed at wavelengths of 633nm and 1539nm. TRIM98 is used to simulate the damage profile caused by the implantation process. It is found that the refractive index change may be mainly due to the damage induced by the nuclear energy loss.