Harmonics from relativistic laser driven plasma surfaces is a prospective high energy attosecond light source infuture XUV pump-probe experiments. Among all the schemes, the most ecient and direct way to realize anisolated attosecond pulse is through using a few-cycle laser as the driving pulse. The two goodness criteria: thelaser to harmonics energy conversion eciency and the \purity" of an isolated attosecond pulse are generallydetermined by a combination of interaction parameters. Through using particle-in-cell simulations and relativisticelectron dynamics model analyses, we explain how these two criteria are a ected by the laser intensity, incidenceangle, carrier-envelope phase, and the plasma scale length. We found that, there exist an optimal plasma scalelength and an optimal incidence angle to eciently generate harmonics and intense attosecond light pulses.When other parameters are xed, using a moderately intense relativistic laser or using a large incidence anglecould result in a higher isolation degree as well as a broader range of controlling parameters to realize an isolatedattosecond light pulse.