Abstract: There is a contradiction between the accuracy and practicability of the short circuit ratio (SCR) for system strength evaluation of renewable energy systems, and numerous construction methods lead to inconsistent values of the critical short circuit ratio (CSCR). This paper analyzed the SCR index and proposed the CSCR calculation method. Firstly, the short circuit capacity of AC system and the equivalent grid-connected capacity of the renewable energy were derived. Secondly, the SCR-S calculated by the capacity was proposed according to the SCR concept, and the SCR-U calculated by the voltage was proposed by the relationship between the SCR and the node voltage. And then, the CSCR was solved according to the maximum transmission power. It was recommended that the CSCR extreme value 2 serve as the basis to divide the system strength. Finally, the simulations verified the accuracy and rationality of the system strength evaluation according to SCR-S, SCR-U and CSCR.