By employing a continuous-wave broadband diode laser, a Self-mixing Cavity Ring-down (SM-CRD) technique is investigated for accurate reflectivity measurement of optical mirrors at 1064nm. The self-mixing effect is induced from the optical feedback, directly reflected from the Ring-down Cavity (RDC), is re-injected into the oscillator cavity of the diode laser. The influence of the self-mixing effect on both output power and spectrum of the diode laser is presented. The coupling efficiency of the laser power into the RDC is greatly enhanced by the self-mixing effect. In the SM-CRD experimental setup, the high reflectivity of the cavity mirrors is measured to be 99.914% and the measurement accuracy is at the 10-5 level. The experimental results demonstrate that the SM-CRD technique is a very simple, inexpensive, precise and stable for high reflectivity measurement.