The calculation program of ASE for a large KrF amplifier with full-3D and quasi-3D code is designed.Comparison between two models is Performed.The ASE and extraction efficiency for a series of aspect ratio, gain to loss ratio and input intensity are calculated. With a 100 J KrF laser made in China as an amplifier,the noise to signal ratio is about 5% under 0.1 saturation inpllt intensity. Our full- 3D and quasi-3D codes are suited to the design of KrF lasers.