Similar to the simplest differential equation of stock price, a set of simultaneous differential equations of stock prices of the same share in both A and H stock markets have been established. This is a set of simultaneous nonlinear differential equations, which can be solved by iteration method via a proof by g-contraction mapping theorem. Further more, the exact solution for equilibrium state and an example of checking the price prediction of "China Petroleum" (601857) at a conference held in May 2008 are given.