In the analysis of HT-7 superconductive tokamak which consists of iron core, thick copper shell and multi-coupling coils system, the electromagnetic parameters are linearized by a novel method; the copper shell is replaced by an equivalent pair of coils in series-opposing; and the multi-coupling is compensated by an intermediate state variable. As a result, the plasma control equation can be set up and solved more simply and easily. The feed-back control system has been designed and rather good results were obtained in experiment. This feed-back control system has high response speed, less computing time and is easy to carry out.