Based on the discrete algorithm of the rate-equation, analysis of the dynamic behavior of Er3 + /Yb3 + co2 doped double-clad fiber amp lifier is achieved. The transient behavior of the single output pulse under different peak input pulse and pump powers, the output power and gain changes of the amp lifierwith only one channel under a train of output pulses, and thatwith two channels under asynchronous transfermode (ATM) are studied. The results show the changes of the output pulse powers and gainswith time are different. When single pulse is injected into the amp li-fier, the peak power of the output pulse depends on the powers of both the peak input pulse and the pump. When a train of input pulses is injected, the output power and gain of the amp lifier reduce gradually before dynamic equilibri-um. When two-channel pulse is injected under asynchronous transfermode, the changes of the output pulse power and gain at overlapp ing time are faster than that of only single-channel output pulse power and gain.