Abstract: With high share of renewable energy sources and large capacity of DC transmission projects, it is urgent to conduct further research on the potential off-grid-risk of renewable energy raised by the transient voltage safety. This paper is to study the coupling mechanism of rotor angle swing and renewable energy temporary power frequency overvoltage. First, for a double-machine equivalent system, the voltage fluctuation characteristics of rotor angle swing process is analyzed by equal area criterion and voltage vector method, and analytic expression of the machine-terminal peak voltage is derived before and after renewable energy connecting to the system. Then, the key factors which affects the peak voltage are extracted, the preset maximum peak value constraint is utilized to obtain the limit value considering the affected factors, and conservatism of the limit value calculation is analyzed combined with practical engineering. Finally, the countermeasures are proposed to improve the overvoltage level of renewable energy, and these suggestions are used to conduct the safe and reliable of power delivery. The theoretical analysis is validated by the time domain simulation on the photovoltaic-thermal-bundled system and the practical system.