The rise and decay curves of Er3+2H11/2 and 4S3/2 upconversion emission in oxyfluoride tellurite glass were measured under the excitation of 808 nm and 980 nm light from laser diodes driven by square wave. Different rise times under 808 nm and 980 nm excitation reveals different excitation paths. Relationship between build-up process of the upconversion emission and lifetime of the intermediate state was analyzed with the rate equations. Excitation processes of the green upconversion emission under 808 nm and 980 nm excitation were confirmed. Square wave excited upconversion emission may also be used for measuring lifetimes of infrared energy levels with a conventional photomultiplier tube.