An end pumped Nd:YAG slab laser amplifier is presented. Low doped and composited Nd:YAG slab is used to obtain continuous wave output power of 330 W and beam qualities of M2x=1.23, M2y=1.73 under the 3 passes amplification stucture. Taking advantage of 4f image system, the loss from light cutoff is reduced and the uniformed energy extraction is realized. In addition, gain coefficients are calculated by the output power of single and multiple pass amplifications. It is also investigated that the multiple amplification structure is good to restrain the amplified spontaneous emission.