We study the law of time evolution of atomic response in an open Λ-type inversi onless lasing system,elucidate the light amplification mechanism without inversi on,and find that the character of the time evolution of atomic response in an open system is obviously different from that in a corresponding closed system. Weconsider that the variation of the oscillation frequency and anharmonic oscillat ion in the process of time evolution originate from the stochastic coherence pro perty of atom oscillators in the system.