A plasma multi-variable control model is presented about Hefei Superconducting Tokamak (HT-7) with iron core and two thick copper shells. The nonlinear electromagnetic parameters produced by the iron core permeability are linearized by a new approach. Each copper shell is equivalent to two pair of coil in opposite series, its contribution for plasma equilibrium and its screening effect to external field can be expressed by self-inductance and mutual-inductance. The control model has been used in HT-7 plasma control system and the good experiment results have been obtained.