Laser indirect-drive has potential to get ultra-high pressure which is very useful for shock-wave physics. A step target is used to get the equation of state (EOS) data under high pressure with laser drive experiment. Since there are preheat effects in step targets, an active velocity interferometer system for any reflector (VISAR) is presented to adjust the thickness change. The thickness change can be obtained after the calculation of fringe changes. The step thickness can be achieved accurately after corrected preheat effect on the thickness. Under conditions for the steady velocity in step target, shock-wave velocity can be gained from experimental data. This technique is suitable for the shock-wave experiment with radiation drive, which provides data of the shock-wave velocity for equation of state with high accuracy.