Absorption spectra, emission spectra, excitation spectra, and upconversion luminescence of Er3+, Tm3+ co-doped NaY(WO4)2 crystal were measured at room temperature. Intensity parameters are calculated from absorption spectra in terms of Judd-Offelt theory. Transition processes of the energy levels of Er3+ and Tm3+ were analyzed in details and the cross relaxations: 1G4(Tm3+)＋4I15/2(Er3+)→3F4(Tm3+) +4F9/2(Er3+) ,1G4(Tm3+)＋4I15/2(Er3+)→3H5(Tm3+)＋4I9/2(Er3+), 3F4(Tm3+)＋4I9/2(Er3+)→3H6(Tm3+)＋4S3/2 or 2H11/2(Er3+) and 4F9/2(Er3+)＋3H6(Tm3+)→4I15/2(Er3+)+3F3(Tm3+) were put forward. Sufficient proofs have been found to illustrate the use of Er3+ as a sensitizer which is effective for the Tm3+-doped crystal.