Dynamic changing process is caused by heat and mass transfer between thermodynamic system and outside, which is not entirely judged in Traditional thermodynamics. Drawing on Modern Thermodynamic and Shell Theory, a new thermodynamic system has been conducted, which consists of inner system, border (shell) and outside. Shell is formed by shell wall and shell gates, and shell wall is insulated. On the basis of above, the opening degree and exchange rate, and the relationship between them are proposed. In the new system, the message as a physical quantity is equivalent to material and energy. The exchanging physical quantities include information, material and energy between thermodynamic system and outside. As the material flow exchange process, stability criterion (border opening degree) is derived. Traditional thermodynamics is perfected and supplemented by the research findings, which offered theory basis for thermodynamic system exact analysis.