The technique of measuring the shock wave velocity directly in the transparent material under the ultra-high pressure is introduced. The preheat effect on shock wave velecity calculation in the indirect driven experiment is studied with imaging velocity interferometer system for any reflector (IVISAR). After analyzing the beginning time of preheat effect,it is found that the shock wave velocity cannot be deduced directly from the experimental data when the radiation temperature is higher than 160 eV with IVISAR. From the experimental data,the starting point of preheat effect is consistent with the laser pulse,and it is found that the preheat effect is the worst when the intensity of laser pulse is the highest. The preheat effect recovers after the intensity of laser pulse decreases. For the experimental data above 160 eV,the shock velocity curve can be calculated by extrapolating the background stripe to the jumping edge. This method is suitable for processing experimental data gotten from the indirectly driven square wave laser and the inorganic transparent window.