To promote the optimal allocation of energy in southern region by marketization and balance the surplus and lack of power resources among southern provinces, based on construction situation of electric power markets in southern provinces, a design for basic rules of the medium and long term power transaction of power grid in southern region of China is proposed, which are inline with the construction of unified electric power market. The details of the design include market management, medium and long term trading mechanisms, connection mechanisms and transaction settlement mechanisms adapted to the spot market. According to the economic development structure and energy distribution characteristics in southern region, this design can solve the increasingly prominent contradiction between reginal power supply and demand to the greatest extent. The problem that the medium and long term electric transaction models of the provinces in southern region are different and difficult to integrate is also solved.