Recently Zeng Wen-ping has proposed a factor to be determined, in other words, using a difference equation contained a number of parameters in approximation to the differential equation. These parameters of the equation can be obtained by error analysis. Zeng Wen-ping got many high accuracy difference schemes; however, some of them are three level and conditionally stable difference scheme. It is hard to construct an unconditionally stable and high precision scheme. The classic Crank-Nicholson scheme is unconditionally stable but its accuracy is too low. At the same time the Sub-domain Precise Integration method was first introduced by Zhong Wan-xie for solving the partial differential equation in 1995. Later many scholars utilized the Sub-domain Precise Integration to solve the convection equation and convection-diffusion equation. They obtained a lot of unconditionally stable difference schemes. Very recently parabolic equation is constructed based on the Sub-domain Precise Integration method in time direction. The difference scheme is five-point and two level implicit schemes. The coefficient matrix of this difference equation is the strict diagonally dominant it can be solved by the square root. Stability analysis of this scheme has been carried out by Fourier. It is shown that this scheme is unconditionally stable and the local truncation error is O(α(Δt)2+α2(Δt)3+ (Δx)4).It is shown by both error analysis and numerical examples that the accuracy of the present method is much better than the classical Crank-Nicholson method and Saul’ev method in[1] ， and the accuracy and stability of the present method is much better than the explicit scheme in [5].Therefore, the difference format is effective and has a good Practicality. The numerical experiments at the end of this paper have shown that the numerical results are in agreement with the theoretical analysis.