To improve the detection sensitivity in the conditions of weak absorption and/or short absorption path, a highly sensitive spectroscopy, namely magnetic rotation cavity enhanced absorption spectroscopy, was developed by combining cavity enhancement with magnetic rotation techniques together, where a ring shape external cavity was adopted as the signal enhancement to avoid the perturbation of the return beam to the laser source. The qualifications of mode matching together with the dependence of the enhanced factor on the mirrors' reflectivity and the cavity loss were presented; moreover, the process of the spectroscopic signal in the experiments was also discussed. As an example, this spectroscopy was employed to observe the spectrum of the triply forbidden transition of O2, and the minimum detectable absorption was measured to be about 4.5×10-8 (1 s integrated time) with 95% coupling efficiency, 48 finess and 13% total loss of the cavity.