The characteristics of pulsed CO2 laser produced Sn plasma plume expansion under ambient air pressures are studied by shadowgraph technique. The dependences of Sn plume expansion position and plasma debris energy on delay time are obtained. The modified drag diffusion model is employed to fit the experimental data. Plasmas are generated by irradiating planar Sn targets using 400 mJ, 75 ns pulses from a CO2 laser. Our results show that the estimated expansion velocity of the plasma in the early stage (delay time is less than 100 ns) is almost 3 cm/μs, and decreases rapidly with delay time to about 0.3 cm/μs at later stage (delay time is larger than 800 ns) because of the thermalization collisions with buffer gas pressure of 1000 Pa. Fitting results of modified drag diffusion model indicate that the plasma plume maximum size is xst=15.2 mm, which is found to agree fairly well with the measured data of 16 mm.