Some analysis of the transient response of the Yb-doped fiber amplifier are performed by solving a set of time-dependent rate and power transfer equations based on finite-difference method. Meanwhile, the variation of time to reach the steady state for upper level population distribution, the forward and backward amplified spontaneous emissions (ASEs) and stored energy on the system parameters including pump power, fiber length, Yb-doped concentration, and core area are numerically simulated, respectively. The results show that, by optimizing pump pulse width, stored energy can reach or even exceed the steady state value of continuous wave (CW) pump. By increasing Yb-doped concentration and core area, stored energy is increased, the ASE is suppressed and the ASE built-up time is postponed. In addition, the experimental results show the validity of the theoretical ASE built-up time. The obtained results can provide important guiding for the optimization of pump pulse width and fiber parameters.OCIS The high energy pulses with millijoule (mJ) level and nanosecond order duration are required for many practical applications such as atmospheric optical communication, laser range finding, remote sensing, lidar, and nonlinear frequency conversion. Compared with Q switching and mode locking methods, a semiconductor seed source followed by a multi-stage fiber master oscillator power amplifier (MOPA) system would be an effective technology to obtain the high energy pulse [1−3] . In the low-repetition-rate high-energy MOPA system, it is very important to optimize the pump pulse width in order to achieve the best amplification results, meanwhile, suppress the amplified spontaneous emission (ASE) and reduce the heat generated in the amplification process [4,5] . So, it is necessary to analyse the time to reach the steady state for stored energy, and the built-up time of ASE. A few theoretical models for Yb-doped fiber amplifiers have been proposed. Hardy et al. deduced a set of approximate analytical expressions to describe the steady state signals, ASE, and Rayleigh scattering, for modeling of high power double-clad fiber amplifiers [6−8] . Wang et al. analysed the dynamic characteristics of high power pulse amplification and Raman effect in detail [9,10] . Chang et al. comparatively studied the small signal pulse and high power pulse amplification [11] . Xu et al. introduced the influence of the pump power, fiber length, and core diameter to the ASE power [12] . However, in the previous researches, before inputting the signal, amplifiers are assumed to have achieved the steady state under continuous wave (CW) pump. Furthermore, the theoretical and experimental study of time to reach the steady state for stored energy and the ASE built-up time of the Yb-doped fiber amplifier have not been reported in the literature.The focus of this letter is the transient response of stored energy and ASE before injecting the seed light. By solving a set of time-dependent rate and power transfer equations based on finite-difference metho...