By using SH of a Q-switched Nd:YAG laser as the exciting pulse with FWHM of 10 ns, the time character of the backward waves were demonstrated for both transparent and absorbing media. The experimental results show that when the duration of the exciting pulses is considerably greater than the characteris tic time T = ∧/v(where A is the wavelength of the laser-induced acoustic wave; v is the sound velocity), the oscillating property of the backward wave disappears, it decays exponentially. The experimental results have been discussed.