To diagnose single- and double-switch open-circuit faults in cascaded H-bridge (CHB) photovoltaic (PV) grid-connected inverters, an open-circuit fault diagnosis method based on an adaptive sliding mode observer is proposed. First, an adaptive sliding mode observer is designed using the mixed logic dynamic (MLD) model of the CHB PV inverter, and fault detection variables are constructed based on actual and estimated currents. Second, voltage thresholds derived from the DC-side capacitor voltage are used for fault module localization. Then, fault localization variables are constructed from current residuals, and the variations of current residuals of the adaptive sliding mode observer under fault conditions are compared to distinguish diagonal switch faults. Finally, double-switch open-circuit fault diagnosis is realized by designing an adaptive sliding mode observer based on the previous fault state. Experiment results show that the proposed method can accurately identify single- and double-switch faults at any position within one fundamental cycle, with fast diagnosis speed and strong robustness.