  The phase transition is both thermodynamically and quantum-mechanically
ubiquitous in nature or laboratory and its understanding is still one of most
active issues in modern physics and related disciplines. The Landau's theory
provides a general framework to describe \textit{phenomenologically} the phase
transition by the introduction of order parameters and the associated symmetry
breakings; and is also taken as starting point to explore the critical
phenomena in connection with phase transitions in renormalization group, which
provides a complete theoretical description of the behavior close to the
critical points. In this sense the microscopic mechanism of the phase
transition remains still to be uncovered. Here we make a first attempt to
explore the microscopic mechanism of the superradiant phase transition in the
quantum Rabi model (QRM). We firstly perform a diagonalization in an operator
space to obtain three fundamental patterns involved in the QRM and then analyze
explicitly their energy evolutions with increasing coupling strengths. The
characteristic behaviors found uncover the microscipic mechanism of the
superradiant phase transition: one is active to drive the happening of phase
transition, the second responses rapidly to the change of the active pattern
and wakes up the third pattern to stablize the new phase. This kind of
dissecting mechanism explains for the first time why and how happens the
superradiant phase transition in the QRM and paves a way to explore the
microscopic mechanism of the phase transitions happening popularly in nature.
