Analytic resolution for relative energy distribution at focal plane of low-order modes for Gaussian laser beams is given. The results are compared with the focal properties at far-field of single mode light for unstable cavity with different magnification M. With. 80% of total laser energy at focal plane as the united criterion of beam divergence, calculation indicates that with stable cavity for mode-selection, one can obtain good beam quality which may be close, even superior, to that for mode-selection for unstable cavity under some conditions. Moreover, the stable cavity construction is simple, the cavity is more stable and the electro-optic efficiency is high as well.