Based on the all-optical pulse Yb-doped double-clad amplifier modeling, studies the dynamic transmission characteristics of nanosecond pulses. Without considering the input signal, the upper-level population of ytterbium-doped double-clad fiber by forward-pumped distributes the characteristics of high intermediate and low at two ends. Considering the continuous signal input and the existence of weak feedback in amplifier, the backward amplified signals caused by the weak feedback consumes more number of upper-level particles as the pump power is higher. When the fiber length increases, a positive signal output power reduces.