Through the introduction of effective reflectivity, the influence of external optical feedback on the oscillating characteristics of a semiconductor laser is discussed, such as the threshold gain, laser frequency and output power, according to the self-consistency condition and the photon and carrier density ratio equation. In the experiment, the threshold current of a laser diode decreased from 420mA to 370mA with external cavity feedback and the slope efficiency increased. The output power obtained by theoretical simulation agrees well with the experiment results.