With the efficient development of the multi-million-kilowatt new energy bases in the northwest region, the landing points of the DC transmission at the receiving end are no longer limited to asynchronous systems. For instance, in the UHVDC project from Inner Mengxi-Jingjinji, both the sending and receiving ends are within the synchronous grid, resulting in a coexistence of AC and DC transmission in the Inner Mongolia West transmission channel. During the simulation calculation of the system design for the ±800 kV UHVDC transmission project from Inner Mengxi-Jingjinji, it was found that the AC and DC transmission capacities have a coupled and mutually exclusive (seesaw) characteristic. This paper focuses on the transmission capacity of the strong DC and weak AC coupled channel, taking the power angle stability as the criterion, and analyzes the impact of adjusting the DC transmission power and the AC transmission power on the overall transmission capacity of AC and DC. The research results show that when the AC channel is too weak to support the DC, maintaining the AC channel to transmit a relatively large amount of power within the limit range and appropriately reducing the DC transmission power is more beneficial to the overall transmission capacity of AC and DC. When the AC channel is strong, the transmission capacity of both the AC and DC channels can be fully utilized.