Passive anti-islanding protections have been widely used in field, but all of them have dead zone of protection operation. On this account, islanding effect cannot completely be distinguished from transient voltage sags. Therefore, the anti-islanding protection cannot effectively cooperate with the low voltage traversing control of photovoltaics. In this paper, the mechanism of islanding is analyzed, and the applicability of passive anti-islanding protections based on voltage and frequency detection is evaluated. The transient characteristics of voltage and frequency during the process from fault to islanding are analyzed by establishing a PSCAD-based photovoltaic islanding simulation model. The relationship between the operation characteristics of field protection and islanding effect is revealed, and the protection countermeasures based on traditional distribution network are proposed. This work is supported by National Key Research and Development Program of China (No. 2017YFB0902900) and Science and Technology Project of State Grid Anhui Electric Power Co., Ltd. (No. 52120017001G).