Noise accumulation of cascade EDFAs and system power budget in a WDM system have been studied. It is found that noise accumulation linearly increases with the number of EDFA

and exponentially increases with lose among EDFAs. When the distance of a system is fixed

the OSNR of a system with lower gain and more series is higher than a system with higher gain and less series .To ensure the characteristic of error code

OSNR of PA should be higher than a value which is directly proportional ratio to code velocity.