Abstract: The capacity of power supplied by external for receiving-end grid is comprehensive reflection of how much power of ‘send to’ and ’come down’. To a certain extent, the capacity of power supplied by external is the sign of the mature development of the power grid. The mature power grid, such as Beijing and Shanghai power grid, can receive massive external electric power. At the same time, it can guarantee the safe and stable operation of the power grid. But there is no clear method to evaluate and improve the capacity of power supplied by external so far. The influence mechanism of capacity of power supplied by external is researched deeply from ‘send to ’and ’come down’. At first, the electric channel of the receiving power grid directly determines the limit of the access to external power of the receiving power grid, which is a prerequisite for the ‘send to’ of large-scale foreign power.Secondly, the internal situation of the power grid, such as the safety and stability of the main grid, the load and the local power supply, are the key factors for the large-scale external power. The evaluation method is proposed based on double optimization with full consideration of security and stability of receiving-end grid. The power optimization of power channel is optimized as the outer layer, and the internal flow optimization is optimized as the inner layer. Based on the proposed evaluation method, the capacity of power supplied by external for Tianjin power grid is 4510MW, corresponding to 29% of most outside their electricity (the actual maximum proportion of Tianjin power grid in 2017 is 26%).It is analyzed that the key factor that restricts the capacity of power supplied by external for Tianjin power grid is: (1)The 500kV transformer capacity is insufficient, restricting the capacity of power supplied by external for Tianjin power grid. Especially, the western power grid of the city does not meet the operation requirements of n-1 during the heavy load period. (2) The 500kV network frame is relatively weak, which affects the evacuation and transfer capacity of the tidal current under the fault mode. Especially the main channel of the eastern power grid in the city is still a single ring network structure. Finally, some problems of receiving external power for receiving-end grid are pointed out. For example: (1)To accept the external power of the power grid, it is necessary to respect the objective reality of the power grid, and make sure to improve blindly. (2)Adhere to the principle of unified planning, and the external electrical channel planning is coordinated with the internal planning of the receiving network. (3)It is necessary to carry out the research on dynamic reactive voltage support of the terminal network. (4)The urban receiving power grid generally has the characteristics of small area, more cloth points and high short circuit. The circular grid is advantageous to the inflow of external power, which is a direction of the development of large scale power grid.