A new oxyfluoride glass system was developed with molar fractions of n(SiO2)=0.30,n(PbF2)=0.50,n=(Al2O3)=0.15,n(AlF3)=(0.049-x),n(TmF3)=y,n(YbF3)=x(x=0,0.001,0.010,0.015,0.020,y=0,0.001) and its upconversion emission properties were investigated. Furthermore the upconversion mechanism was discussed. The experimental results showed that there was not upconversion emission for Tm3+ singly doped glasses under 970 nm excitation. While strong blue (452 nm, 476 nm), red (647 nm) and near infrared (971 nm) emissions were found in Tm3+/Yb3+ codoped glasses, which responded to the following radiative transition: 1D2→3F4, 1G4→3H6, 1G4→3F4 and 3H4→3H6. Moreover, the intensity of upconversion luminescence was strongly dependent on the concentration of Yb3+. It was indicated that under 970 nm excitation the upconversion luminescence intensity of Tm3+ increased with Yb3+ sensitizing. With Yb3+ concentration increasing, absorption of pump source and energy transfer between Tm3+ and Yb3+ were both enhanced, and as a result the upconversion luminescence intensity was increased.