A simple cavity ring-down (CRD) technique based on an intensity-modulated continuous-wave (CW) diode laser is developed to determine simultaneously the reflectivities of the cavity mirror and the inserted planar mirror. The straight- and folded-cavity configurations with a same cavity length are established. Under both cavity configurations and with phase-locked technique, the amplitude and phase-shift of the first harmonic of the CRD signal are measured as a function of the modulation frequency covering an appropriate range. Effects of the system frequency response are eliminated in the data processing, and then the amplitude ratio and phase-shift difference between the two cavity configurations are fitted to an explicit theory to determine the reflectivities. The measurements are repeated with two cavity lengths, one of which is a 100 cm confocal cavity and the other a 110 cm stable cavity. The reflectivities of the cavity mirror and the inserted mirror are measured to be 99.784% and 99.668%, respectively, with an uncertainty in the order of 10-5.