Because of the different power flow directions during the charge and discharge modes of energy storage systems (ESS), the fault characteristics of PV-energy storage power stations (PVESPs) differ significantly from those of synchronous generators and standalone PV power stations. As a result, traditional current differential protection faces a higher risk of malfunction. This paper focuses on AC-coupled PVESPs with shared ESS. It first derives and analyzes the phase angle variation characteristics of the short-circuit current from the ESS under both charge and discharge modes, along with their corresponding calculation formulas. Further analysis of the short-circuit current behavior in AC-coupled PVESPs reveals the reasons why traditional current differential protection may malfunction for the station operating as a power source and as a load, respectively. To address these issues, a novel pilot protection criterion is proposed based on adaptive adjustment of the equivalent impedance amplitude ratio. By using the equivalent impedance ratio of the two sides of the line, the method distinguishes between the internal and external faults as well as normal operating conditions. Also, an adaptive adjustment coefficient is added to amplify the difference between the equivalent impedances of the two sides under internal faults, ensuring that the proposed protection mechanism can still operate reliably even during high-resistance faults in lines with a high-proportion of PVESPS. Finally, an electromagnetic transient model of a power grid with AC-coupled PVESPs is built in PSCAD/EMTDC, and the operational characteristics of the proposed protection principle are verified.