In this paper, the general optical Bloch equation describing interaction between intensive fields and two level system is obtained without using rotating wave approximation. The first-order solution of this equation is obtained by iterative method and analytic formulas of transition probability as a function of time, perturbation frequency and strength are presented. These results are in agreement with those obtained by an iterative step-wise power series and numerical integration solution for the time dependent Schrodnger equation. And these simplified and clear mathmatical formulas facilitate the application of physics.