Laser indirect-drive has the potential to get ultra-high pressure which is very useful for shock physics. The isentropic compression experiment with long pulse in laser direct-drive is very sensitive to the laser intensity variation. The important technique for isentropic compression experiment with long pulse in laser direct-drive has been introduced with the diagnosis of free surface velocity. The improvement way can be obtained from the experimental data. The target, experimental data and the important technique can be analyzed. The full velocity history of free surface can be achieved after design and control the laser pulse with the comparison of experiment data under 1000 J laser energy. The signal of free surface disappears when the free surface velocity reaches 11.3 km/s. After comparing the simulation data with Multi-1D program and experimental result, the complex of shock wave system in Al sample has been shown because there is no confinement on the free surface. However, this technology is useful to study the main shock wave in the sample. When the sample with transparent window is difficult to manufacture, these experimental data show the technology for the further development of isentropic compression experiment with long pulse in laser direct-drive.