Upconversion blue emissions of Tm3+ ions with different doping (0.5at.%, 3 at.%, 5 at.%) in NaY(WO4)2 crystals are investigated under 800nm diode laser excitation. The upconversion mechanism and the dependence of upconversion efficiency on ion concentration are analyzed with respect to the absorption spectra, fluorescence spectra, and parameters calculated by Judd-Ofelt theory. Four ion-ion interaction processes: 3H5+1G4→3H6+1D2, 3H5+3H5→3H6+3F3, 1G4+3H6→3F4+3F3,1G4+3H6→3F3+3F4, which influence the upconversion dominantly in Tm3+ heavy-doped system, are demonstrated theoretically and experimentally. The results indicate that cross-relaxation and cooperative upconversion are the dominant processes affecting the blue emission efficiency.