In this paper, a new idea is proposed, using two driven laser beams combined one beam in prime frequency with another beam in double frequency to shot on target obliquely. According to theoretical calculation, not only energy loss due to the double frequency can be avoid but also a higher gain coefficient can be achieved, comparing to use two driven laser beams in double frequency with same power in the prime frequency. A design of long distance four targets series coupling is also presented, the X-ray laser produced by former two targets is collected by a pair of junction mirrors with grazing incidence, then amplified by latter two targets. The theoretical research predicts that, taking advantage of this method, saturated gain of X-ray laser at spectral "water window" can be got by Shen-Guang II facility.