The transient response of donor fluorescence under the influence of energy transfer has been analyzed using a non-linear model including the back-transfer effect of energy from acceptors to donors. In addition to the non-linear product of donor and acceptor populations and their improved distribution functions, the back-transfer process turned out to be important in describing the interacting donor-acceptor system. It was revealed that the back-transfer of energy enhances the non-exponentiality of decay curves. The numerical analysis of energy transfer in Tb3+-doped glasses showed a good agreement between experimental results and the model, which enables us to discuss the more detailed mechanism of energy transfer.