Atomic clock is an extremely precise oscillator,but the increase of precision of it is limited by the noise in it.The period and chaos in an atomic clock is simulated by using a type of Duffing equation.Statistical characteristis of various chaos are discussed.According to the Lyapunov exponent of the atomic clock signal,the chaos in the clock and its motivational mechanism is analyzed.Measured data from CSAO Laboratory during 1994.6.1—8.20 are processed and statistical characteristics of chaos described.