  The active asteroid 311P is one of the two targets of a planned Chinese
asteroid exploration mission Tianwen-2. During 2013, 311P experienced several
mass-loss events and exhibited multiple comet-like tails. Here we analyze the
morphology and surface brightness of the tails to investigate the dust
environment around the nucleus and mechanism of activities. We enhance the
features of the tails using image processing techniques to obtain information
about the morphology of the tails, and fit processed images to the
syndyne-synchrone diagrams. The fitting results give estimations of the upper
limits of the durations ($2\sim8$ days) of eruptions and the dust size range
($0.006\sim38.9$ mm) in the tails. The results of surface photometry performed
for each dust tail show that the brightness distribution index of each tail
ranged from approximately -1.81 to 0 and the dust size distribution indices of
311P's tails ranged from -2.29 to -1.45. The quantity of particles in each tail
ranged from 0.5 to 8 $\times10^6$\,kg, which leads to a total dust-loss
quantity of $3.0\times10^7$\,kg and a mass loss rate of 1.59 kg s$^{-1}$.
Sublimation, continuous impacts or tidal forces of planets are excluded as
explanations for 311P's activities, and rotational instability remains a
possible activation cause without strong evidence against it.
