Abstract: This paper investigates the cloud ignition margin under the condition where the secondary initiation explosive charge is positioned at the periphery of the cloud formed after the dispersion of the fuel-air explosive (FAE). Through a combination of experimental study, empirical formula, and numerical simulation, the research focuses on determining the threshold value of the peak overpressure at the cloud edge. A prototype containing 12.5 kg of FAE was designed, and dispersion tests were conducted to identify the maximum radius of the cloud. A 1 kg-class HMX-based explosive was employed as the secondary initiation explosive charge. Experimental studies were carried out to establish the relationship between the distance of the charge from the cloud edge and the cloud ignition state, ultimately determining a distance threshold. The peak overpressure at the cloud edge was selected as a metric to evaluate the magnitude of the initiation energy required for cloud ignition. Based on an empirical formula and numerical simulations, the threshold value of the peak overpressure at the cloud edge necessary to satisfy the cloud ignition conditions was investigated. Moreover, the peak overpressure was validated in accordance with the critical energy flow criterion. The results demonstrate that positioning a 1 kg-class HMX-based explosive at the periphery of the cloud can also induce detonation of the cloud, provided that the distance from the cloud edge does not exceed 0.5 meters. Furthermore, when the energy of the secondary initiation charge column reaches a level sufficient to enable the cloud to experience stable detonation, the position of the secondary initiation charge column exerts minimal influence on the detonation overpressure. To ensure reliable cloud ignition performance, the edge peak overpressure of the cloud generated by the secondary initiation explosive charge should not be less than 5 MPa. This study considers the stringent conditions required for cloud ignition and provides valuable insights and data for the design of secondary initiation explosive charges. The research findings can serve as a technical foundation for optimizing the performance of secondary initiation systems in practical applications.