Abstract: This paper introduces a theoretical model of the dynamical compilation system of LISP——the computational model RC, which consists of RC machine, RC instructions and RC language. On RC machine RC language can be interpreted directly and can run after compiled. The author gives the operational semantics of RC instructions, discusses formally the two execution modes of RC language and proves that they are equivalent and can be switched freely. Based on this model, DCLISP, a dialect of LISP with the function of dynamical compilation has been developed.