A full-automatic precision energy measurement system on small signal was described in detail, which was used for eight beams' power balance in Shenguang Ⅱ. An assembly attenuator of half-wave plate and polarizers is used to check up the performance of the system. The precision of stability precedes 1% (rms). And the measuring range covers three magnitudes. It enlarges the measuring range of the original measurement facility of Shenguang Ⅱ.