A firing frequency controller which includes a cylinder body, a piston assembly, and a unidirectional seal is provided. The cylinder body includes a first orifice, a second orifice and a third orifice, wherein the gas flux through the first orifice and the third orifice are inequality. The piston assembly disposed inside said cylinder body is repeatedly moved. The unidirectional seal is disposed at outside of the piston and adjacent to the inner wall of the cylinder body. The unidirectional seal is contacted with the cylinder body and is kept airtight while moved at one direction. The unidirectional seal is not contacted with the cylinder body and kept non-airtight while moved at another direction. Because of pressure difference, the velocity of piston assembly can be adjusted and then the firing frequency of toy gun can be controlled.