In this paper, the agitation characteristic curve of medium light particles in stirred tank is proposed based on numerical simulation study. The agitation characteristic curve contains the power number curve as the input of the stirring system and the particle dispersion curve as the output. The standard k-epsilon turbulence model together with the volume of fluid model (VOF) and the discrete phase model (DPM) were used to simulate the particle distribution in an unbaffled tank with free surfaces. Based on the standard deviation, dispersion degree curves in both axial and radial directions (sigma(la), and sigma(lr)) were proposed to quantify the discrete particle distribution in the tank. Compared with the instantaneous fluid field, the dispersion degree curves represent well the particle distribution for different rotation speeds and immergences. Besides, the power number (N-p) was used to quantify the power consumed during the drawdown process of floating particles. This work shows that the agitation characteristic curve would serve as a benchmark so that stirring systems with different configurations could be compared and optimized.