A novel and more realistic calculation model and method for multipass laser pulse amplification has been put forward based upon the double side pumping scheme. We present here the systematic calculation and analysis on the amplification characteristics of the double side pumped confocal cavity Ti:sapphire amplifier and the chirped pulse multipass amplification through a new amplification configuration. Our experimental results show good agreement with the calculations. With such amplifier,the pass number is noticably reduced and some electro optical elements in regenerative amplifiers have been omitted.