Abstract: In order to prevent the laser beam from being refracted out of lasing medium, we proposed and carried out a novel design named double -target opposing coupling early 1990. We developed opposing coupling idea and proposed following coupling idea for four -target series coupling experiments early 1991. The total length for four targets is up to 5.6cm. We have developed a series of codes, which can simulate the laser-produced plasma condition, population inversion, gain of laser lines , and propagation and amplification of laser lines in lasing medium. Before these four-target series coupling experiments, we adjusted some parameters in JB19 and ALPHA codes in order to match the results of last double -target opposing coupling experiments. The results of simulation for laser line of 23.2nm agree with those of the last round experiment. Then we simulate this round of experiment and get the parameters of the distance and angle between the targets in order to get best coupling. The results of the experiments show that the simulation is good. The gain length product（GL） of small signal is up to about 18 for both lines at 23.2 and 23.6 nm, and the effective GL is 16.4 and 15.7 for these two lines, respectively. These two lines are obviously tending to saturation. The divergence of laser beams reduces to 3 ⁴ mrad.