Equation of state (EOS) experiments were performed using a double-frequency (2ω) laser pulse, which exported from the ninth beam of “SG-Ⅱ” laser facility. The ninth laser beam was smoothed by lens-array (LA) uniform illumination technique to generate a flat-top intensity distribution at the target plane and planar shock fronts in targets, which was validated by the plane-target experiments. The shockwave stabilities were experimentally measured by the multi-step target technique. The results showed that the shocks were steady enough during the target thickness range in which the shock velocity was measured. Also, the results were consistent with the numerical simulation results. On this base, the Au EOS was measured by using the impedance-matching technique. Shock pressure higher than 2.7 TPa was achieved in Au, and the relative expanded uncertainties of the measured shockwave velocity were 2% (K=2). Moreover, the experimental data were compared with existing experimental data and EOS models.