A method for producing an externally injected gain switch laser ultrashort pulse, comprising the following steps of ultrashort light pulse signals having multi-longitudinal mode characteristic produced by the gain switch laser are inputted into an optical amplifier and then amplified; a spectral component signal selector selects a narrow spectral component signal outputted by the optical amplifier, the narrow spectral component signal is within an amplified spontaneous emission noise frequency band and its central wavelength is equal to the longitudinal mode of the gain switch laser; a route of the narrow spectral component signal is used as an external seed light and reinjected into the gain switch laser via a spectral component signal feedback loop. Therefore, the oscillation of a selected single longitudinal mode within the cavity of the gain switch laser is enhanced, thereby forming an externally light injected locking.