Abstract: The strength of system with renewable energy generation (REG) has an important impact on the severity of transient overvoltage. A quantification method is proposed by using the short-circuit ratio (SCR) as the system strength index and considering the interaction in multi-infeed systems. First, the relationship is analyzed among the voltage, reactive power, and short-circuit capacity at the transient overvoltage moment. Furthermore, the interaction in multi-infeed systems is analyzed by using voltage phase method, and the mathematical model for multi-infeed system is established. The correlation between the system strength and the transient overvoltage is analyzed by combining SCR and this model. Meanwhile, a quantitative method of REG maximum scale is proposed by taking 1.3pu as the constraint due to the transient overvoltage, and the calculation method of minimum short-circuit capacity is proposed to improve system strength. Finally, the correlation between system strength and transient overvoltage is verified by examples.