  Economic systems are similar with physic systems for their large number of
individuals and the exist of equilibrium. In this paper, we present a model
applying the equilibrium statistical model in economic systems. Consistent with
statistical physics, we define a series of concepts, such as economic
temperature, economic pressure, economic potential, wealth and population.
Moreover, we suggest that these parameters show pretty close relationship with
the concepts in economy. This paper presents related concepts in the
equilibrium economic model and constructs significant theorems and corollaries,
which is derived from the priori possibility postulate, getting theorems
including the equilibrium theorem between open systems, the distribution
theorem of wealth and population along with related corollaries. More
importantly, we demonstrate a method constructing optimal density of states,
optimizing a macroscopic parameter depending on need to get the distribution of
density of states utilizing the variation method, which is significant for the
development of a society. In addition, we calculate a simple economic system as
an example, indicating that the system occupied mostly by the middle class
could develop stably and soundly, explaining the reason for resulting
distributions of macroscopic parameters.
