Carrying out the direct power purchase for large users can stimulate the power market potential, which is the breakthrough of China's power market-oriented reform. The electricity pricing method is a key factor affecting the development of power market. The existing annual cost accounting method of the power transmission and distribution cannot reflect the influence on transmission cost when users choose different power stations to purchase electricity and use it in different time periods. This paper proposes dynamic electricity price model based on time-space risk factors for the direct power purchase of large users to reflect the situation of different risk because of the atmosphere and environment in which power grid encountered. The power transmission and distribution costs will be decomposed into two categories, thus to determine the monthly calculation of dynamic electricity price method. By constructing a dynamic electricity price model, users can select the low-risk month of multi-power consumption according to their own characteristics and reduce the transmission pressure of the high-risk period of the power grid to improve grid reliability. The calculation results show that the model is effective and practical, and it can be used to provide reference for estimating the price of power transmission and distribution for government. This work is supported by Major Basic Foresight Science and Technology of State Grid Corporation of China (No. SG20141187).