Transmission capacity and operation stability are affected by reactive power and voltage control modes for ultrahigh voltage(UHV) transmission line. This paper presents the powerangle characteristic equations described by the distributed parameters and powervoltage characteristic equations for transmission line. Using powervoltage characteristic equations, it analyzes operation characteristic under constant voltage control at the sending end and constant power factor at the receiving end of transmission line. At the same time, it draws PV characteristic curves under three typical power factors at the receiving end. It indicates transmission capacity of transmission line will increase with power factors at the receiving end rising. But power area leading to overvoltage will extend, especially it appears very big over opencircuit voltage area under leading power factor at the receiving end. Using the powerangle characteristic equations, it derives formulas of surplus reactive power and demands of reactive power compensation capacity under constant voltage of each end of transmission line. Taking JindongnanNanyang Jingmen 1 000 kV UHV transmission line as an example, transmission capacity and demands of reactive power compensation capacity are calculated for two control modes.