The alternate dual-wavelength cavity ring down spectroscopy (DW-CRDS) had been developed for isotope analysis, where the isotopic abundance ratios are simultaneously determined based on two continuous-wave distributed-feedback (DFB) diode lasers operating at 1.550 um and 1.660 um. Alternation of the two DFB lasers was performed by an optical switch via automatically process to couple into a 1.5 m long cavity with finesse>90,000. The DW-CRDS scheme we proposed here is simple and robust. It is easily modified on the basis of conventional CRDS by implanting an extra optical switch, which allows fast laser alternation between two distinct wavelength over a wide bandwidth, sharing the same histogram lock and detection system.