Thulium and ytterbium codoped KY(WO4)2 crystals were prepared by using top seeded solution growth (TSSG) method. Its absorption spectra were recorded in the region of 290～1200 nm at room temperature. The energy transfer mechanism from Yb3+ to Tm3+ ions was analyzed by using up-conversion model and the transitions of energy levels were assigned. The rate equation of Yb3+ ions to Tm3+ ions energy transition was set up. The efficiency of Yb3+ ions to Tm3+ ions energy transition is close to 1. The up-conversion blue fluorescences at 476 nm were measured when the sample was pumped by 974 nm diode laser. The value of emission cross section for 1G4 to 3H6 transition is estimated with Fadenbrug-luechtbauer method and the peak value is about 1.51×10-20 cm2.