Abstract: 19.6nm Nelike Ge Xray laser (XRL) can be used as a source to diagnose RayleighTaylor instability in laser induced plasma. In this paper, systemic optimum designs and theoretical analysis to Nelike Ge XRL driven by premain short pulses were conducted by a series of codes, which had been tested by experiments. Simulation results show that, adopting driven conditions of 2%~3% prepulse , 6~8ns premain pulse interval and 40TW/cm2 power intensity，a gain area of 19.6nm laser line beyond 60μm and gain duration of 90ps can be obtained. Furthermore, with 16mm plane target, the gain gets to 11.8/cm, and if 6mrad/cm curved target is adopted, it reaches to 13.3/cm, and small signal gainlength product gainlength product of single target is about 21.3, which means that saturated gain