The upper-laser-level lifetime (fluorescence lifetime) of the rear earth dopant in the active fiber is a key parameter which indicates the performance of the fiber, and takes an important role in designing the laser system. However, the accurate measurement of fluorescence lifetime in active fiber remains challenging, which mainly rely on the direct measurement of laser induced fluorescence lifetime of the active fiber or lifetime measurement of bulk laser glass. The former method suffers the error due to the amplified spontaneous emission and the reabsorption process, while the latter ignores the influence of high temperature and tension produced during the fiber drawing on the emission behavior of the material. Therefore, the accuracy of these measurements can become a problem. In this work, we propose a new approach to measuring the upper-laser-level lifetime of the rear earth dopant in the active fiber based on the power/energy performance of the fiber amplifier. The population inversion, i. e. the energy storage, in the active fiber of a fiber amplifier is a function of upper-laser-level lifetime. Therefore, the upper-laser-level lifetime can be derived by measuring the average power or output pulse energy of the amplifier, given that the energy storage in the active fiber is extracted adequately by a seed laser. Using the rate equations, we model the population inversion and energy storage in the active fiber each as a function of pump power and time, and the resulting relationship between the upper-laser-level lifetime and the average output power. The upper-laser-level lifetimes of several commercial Yb-doped active fibers are experimentally measured by this method through using the fibers as the gain media of the amplifier operated at 1064 nm. The convenience of experimental data processing is also discussed. The measured lifetime and evolution trend of the lifetime with dopant concentration exhibitthat they are in good agreement with those from other reports and the theoretical model, which verifies the feasibility of this method.