We investigate the behaviour of the intracavity field in a strongly coupled atom–cavity system (a collection of two-level atoms inside a high-Q cavity) driven by a short strong external resonant pulse. A new effect is predicted, the amplification of the intracavity field appears if the atomic number density in a cavity exceeds the instability threshold which is proportional to the square of the strong driving field. The amplification gain is proportional to the amplitude of the strong driving field. The amplitude of the intracavity field in the vicinity of a critical point above the threshold is time-dependent modulated with harmonics of half of the Rabi frequency of the driving field.