This paper demonstrates that the single crystal optical parametric amplification process (OPA) satisfies spectral parity-time (PT) anti-symmetry under specific boundary conditions, and the PT symmetry threshold point exhibits a gain jump property. For an OPA with phase mismatch, the PT symmetry of the system can be controlled by instantaneous adjustment of the pump intensity. Based on this property, this paper constructs an ultrafast optical switch, which can combine with amplitude modulated pump to directly convert continuous laser into an ultrashort output pulse sequence. On the other hand, the optical switch can be used for further pulse compression and is promising to be used as an ultrashort mid-infrared seed source. The proposed scheme is easy to directly generate ultrashort pulse sequence with repetition rate higher than 10 GHz because the optical resonant cavity is not required.