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The energy balance (EB) model of a primary side regulation (PSR) fly‐back converter in the discontinuous conduction mode (DCM) is discussed in this paper. Based on this EB model, the stability of a PSR fly‐back converter in the pulse skipping mode (PSM) is analyzed, and a self‐adapting modulation factor control strategy is proposed. Theoretical analysis and simulation results show that by saving an optocoupler and correlative circuits, which are necessary in traditional PSM fly‐back converters, the modulation factor tolerance controlled by this method is 1.26% on average, corresponding to the ideal value. Compared with traditional fly‐back PSM controllers, the power saved in the sampler/comparator modules is 87% on average for a load range of 1 Ω to 1 kΩ.