Abstract: Richtmyer-Meshkov (R-M) instability has attracted extensive attention in many engineering application fields. Shock tube experiment is a widely used technique in the study of R-M instability. Thanks to the molecular-level traceability, planar laser-induced fluorescence (PLIF) diagnostic technique o ers concentration (mole fraction) map of interface gas with high resolution. It shows the way to study the mixing in the instability evolution. Di use gas cylinder interfaces are accelerated by a weak shock wave (Ma=1:25) in the experiment. Using PLIF, the mixing of interface is investigated, which is induced by R-M instability. By changing the aspect ratio of elliptic cylinder, concentration maps of diffuse gas cylinders with three types of initial configurations are obtained. The simple stretching, the secondary instability and the jet caused by extrusion in the concentration field are clearly revealed. Moreover, the mixing rate of different stages of evolution is calculated from concentration field. Attempting to understand the mechanism of mixing, instantaneous mixing rate, total mixing rate of the interface and the probability density distribution of mixing rate are analyzed in detail. At early time, baroclinic vorticity accelerates the mixing through stretching interface and intensifying concentration gradient. As the evolution develops, the secondary instability appears, causing the flow transitions to turbulence as a result of smallscale convection. At meantime, molecular mixing induced by concentration gradient is weakened. There is a competitory relationship between diffusion caused by concentration gradient and convection caused by secondary instability, which control the mixing together.