A series of experiments on the interaction length between the sound and the light with Raman-Nath diffraction is reported. Then the optimum interaction length at certain sound frequency is defined. Based on those results, the devices of acoustooptic Q-switch, made of H2O or TeO2 as the medium, in terms of Raman-Nath diffraction are respectively designed. The diffraction and the loss of the devices are tested. Finally the device made of TeO2 is employed to modulate a long pulsed Nd:YAG/KTP laser. The modulated pulse width is measured as about 200 ns at the repetition frequency of 10 kHz.