The timing of multiple shock waves is crucial technique to the performance of inertial confinement fusion ignition targets. With the requirement for shock timing, the timing ability of Shen Guang-III prototype is analyzed by experimental data of two shots. The laser pulse in Shen Guang-III prototype is shaped in time scale, and the window material is z-cut quartz. The stream lines of two shock waves in quartz target are presented with Multi-1D program. Two steps of radiation drive two shock waves that coalesce in the quartz target, the resulting history, coalesce times, and transit times are observed in the stream lines. With the experiment results of two shots, the consistence of resulting history and coalesce times of two shock waves is provided under the same condition of laser pulse, target and diagnostic parameter. The differences in shock wave velocity, fringe contrast and quality occur at coalesce times of two shock waves. And the reflectivity of second shock wave does not increase compared with that of first shock wave. These experimental results and method provide the strong support for shock timing technique.