When a shock wave reflects from the free surface of a solid sample, fragments may be emitted from the surface. Understanding the process of the fragments mixing with gas is an important subject for current researches in inertial confinement fusion and high pressure science. Particularly, obtaining the fragments size and distribution is important for developing or validating the physical fragmentation model. At present, the reported quantitative data are less due to the great challenges in the time-resolved measurements of the fragments.#br#Recently, high-power laser has appeared as a promising shock loading means for fragment investigation. The advantages existing in such means mainly include small sample (~μm to mm-order), convenient dynamic diagnosis and soft recovery of fragments. Our group has performed the dynamic fragmentation experiments under laser shock loading metal. The ejected fragments under different loading pressures are softly recovered by low density medium of poly 4-methy1-1-pentene (PMP) foam. The sizes, shapes and penetration depths of the fragments are quantitatively analyzed by X-ray micro-tomography and the improved-watershed method.#br#This paper mainly reports the research advances in the process of the fragments mixing with gas. The laser-driven shock experiments of tin sample are performed at Shenguang-Ⅲ prototype laser facility. Under two typical loading pressures, the fragments mixed with gas (N2) are recovered by PMP foam with a density of 200 mg/cm3, and the pressure of gas is 1 atm. The high resolution reconstructed images of the recovered fragments provided by X-ray micro-tomography and computed tomography reconstruction show that the shapes of the fragments are almost homogeneous, and their sizes are in a range of about 1-20 micron. These images are very different from the images of the fragments recovered in vacuum under similar loading pressures. The observed fragments under loading pressure less than 10 GPa in vacuum are some thin layers, while the loading pressure is increased up to more than 30 GPa, a large number of small spherical particles are observed in the front of the recovery fragments, thin layers in the middle, and these spherical particles have diameters ranging from one dozen to several hundreds of micrometers. The sizes and number of fragments are analyzed by the improved watershed method. The resulting distribution of the fragments mixed with gas follows bilinear exponential distribution. Comprehensive analyses of former simulations and our experimental results show that the secondary fragmentation should occur in the process of the fragments mixing with gas.