In this paper,the theory of an optical wave-guide amplifier is presented,the aggregation effect caused by the small absorption cross-section and exorbitant concentration of Er ions are analyzed and the cooperative upconversion and excited absorption caused by the aggregation effect are considered to be able to limit the gain of a Yb-Er doped phosphate wave-guide amplifier.The incorporation of Yb ions as the sensitizer into the optical wave-guide amplifier can not only reduce the aggregation effect due to the high concentration of Er ions,but also solve the gain problem for the optical wave-guide effectively. On the basis of the cooperative upconversion effect considered,a Longku algorithm is used to study the Yb-Er co-doped optical wave-guide amplifier.Two main factors i.e.Yb ion concentration and wave-guide length are discussed and the optimum wave-guide length and Yb ion doping concentration are calculated.