Abstract Synaptic circuits are highly sensitive to sensory experience during a critical period in early development. The maturation of GABA inhibition in the visual cortex is suggested to be required for both the onset and closure of the critical period for ocular dominance (OD) plasticity, although the underlying mechanism is unclear. This study examines a model of a visual cortical cell to investigate the mechanism by which inhibitory pathway regulates OD plasticity, through the competition between the groups of correlated inputs from two eyes. We show that when feedforward inhibition is at a low level, the activity-dependent competition does not arise. In the lack of competition, synaptic dynamics are monostable, which prevents the sensory experience to be embedded into synaptic weights. When the feedforward inhibition becomes greater than a threshold, the competitive interaction segregates the input groups into dominant and recessive ones. In this case, the synaptic dynamics become bistable, which provides the synaptic pattern with the ability to reflect sensory experience, opening the critical period. When the feedforward inhibition is further increased, a strong stability of synaptic patterns makes it difficult to change according to input stimuli. Therefore, it becomes difficult again for the synaptic weights to reflect the information about sensory stimuli, closing the critical period. Our hypothesis suggests that the start and end of critical period plasticity may be explained by the competitive dynamics of synapses, which is modulated by the feedforward inhibition.