The research on mathematical model of coordinate control system (CCS) of boiler-turbine generator unit in thermal control field has achieved some significant results. But there are still some important theoretical and conceptual issues. And the correctness of every type of mathematical model has a direct effect on its practical application. Such as Bell- Astrom model, the controlled object described by which is different from the large capability unit. It is pointed out that there are some pathological phenomenon in the model of Bell- Astrom through mathematical analysis and simulation testing. The most important issue is that the model of Bell- Astrom is inconsistent with energy conservation law in steady-state conditions of controlled object described by it. Meanwhile, another practical mathematical model is provided in this paper, which is more in keeping with contemporary large capacity unit.