The thermal-mechanics coupling rule and analytic method of coupling based on principle of cellular automation (CA) have been put forward for the sake of solving coupling effect problem of multi-physics field on the milling insert during milling. The evolution rule of stress function was derived on milling insert according to thermal elasticity by analysis of thermal-mechanics coupling of milling insert based on principle of CA. The temperature change function of the milling insert was deduced in the event of fixed cutting parameter based on heat transfer theory. The local rule of CA on thermal-stress of milling insert was established. The influence factor and distribution regularities of thermal stress were obtained. Compared the numerical-value result of analysis on thermal-mechanics coupling based on principle of CA with analytic result of finite element method(FEM), the effectiveness of CA method was testified. A way of analysis with flexibility was provided for coupling problem of thermal stress field and mechanical stress field for milling insert. The theory basis of research was provided for optimum structural design of indexable milling inserts.