The periodic oscillations and clock division in semiconductor laser with external optical injection have been studied theoretically and experimentally. The resuts indicate that double period oscillation due to semiconductor laser with extenal optical injection can lead to clock division of the injected optical pulses We have simulated the time series and power spectrum of output light from semionductor laser by the coupled rate-equation method， and studied a series of periodic oscillations in laser cavity. When the slave laser was drived into the dynamical period_two state with propriety injection optical power， and modulation frequency of injected optical pulses is twice the oscillation frequency， the clock division of injected optical pulses occurs. Moreover， in experiment，we have demonstrated that the output pulses appear at a repetition rate of 3.16GHz when optical pulses with 6.32GHz repetition rate are injected into the Fabry-Perot laser diode. The experimental result is well consistent with theoretical analysis.