Under low vacuum condition (5Pa), Q-switched YAG laser ablates plane aluminum target and plasma are produced. Optical emission spectroscopy is used to carry out time-resolved analysis of atomic particles. Using the resonance transition of AlI 396.1 nm(3p2P—4s2S), the velocity distribution of AlI in space has been determined at laser power densities of 1.91×1010, 5.10×1010 and 7.64×1010W/cm2, respectively, when the ablating size is about 200 μm. The velocity is of the order of 106 cm/s and the time decays nearly agree with the exponential law as the radial distance from the target surface increases. At a fixed position, the velocity shows some decrease as the laser power density being increased, but when the distance is above 15 mm, the velocity converges to unification one. Shockwave model is used and gives a good agreement with the experimental results. From the model, it is deduced that the shockwave front has cylindrical symmetry.