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Researches on UHV power transmission have been carried out more than forty years. It aims to improve the transmitting capability of power grid and realize the long-distance large power transmission and system interconnection. In China, the successful commissioning of UHV pilot projects makes it possible to optimally dispose electric power resources on a large scale, to effectively control the short-circuit current in the areas of high electric density, and to solve the dynamic stability problem in long chained power grid. On the background of UHV networking, the system frequency will be more stable, but even micro fluctuation of system frequency can cause the power flow of UHV tie-line to be changed. Started from instructing the relationship between active power of tie-line and system frequency, this paper researches on the primary cause of why tie-line flow changes and analyzes the principle how it changes. Then new control strategies for tie-line flow control on the background of UHV networking are especially presented, which provides some reference to Three-Hua synchronous UHV power grid, which connects North China, East China and Central China grids.