A simplified optical heterodyne cavity ring down spectroscopy, which is relatively easy to implement and more suitable for molecular ro-vibrational spectrum study, is suggested in the present paper. It avoids measuring the ring down time directly, and in addition cancels the DC common mode signal resulting from the loss of the cavity mirrors. Theoretically, it could reach the quantum noise detection limit. Furthermore, its lineshape of the first derivative of Gaussian profile is theoretically analyzed, and the dependence of the spectral intensity and lineshape on some experimental parameters are also discussed, and thus the optimal parameters are obtained.