A one-dimensional radiation magneto-hydrodynamics code coupled to an atomic kinetics code was developed, whose simulation results were compared with the experimental observations. Based on the simulation results, the physical processes and physical scheme of Z-pinch implosions were studied. During the implosion phase, the plasmas are heated and ionized, and before the stagnation, the greatly ionized plasmas of high temperature and high density are created. Then the continuously rising density leads to the recombination of plasmas and the emission of an intense X-ray pulse. Our simulation shows that there are two necessary conditions for producing high yield of K-shell energy. Firstly, highly ionized plasmas must be prepared before the beginning of recombination. Secondly, the density of the plasmas which are in recombination phase should be high enough to make the plasmas recombine with a large rate so that more internal energy can be drawn before stagnation. As another result of our study, the atomic processes play a role in cooling free electrons during the recombination phase.