  The analysis of light variation of M87 can help us understand the disc
evolution. In the past decade, M87 has experienced several short-term light
variabilities related to flares. We also find there are year-scale X-ray
variations in the core of M87. Their light variability properties are similar
to clumpy-ADAF. By re-analyzing 56 $\it Chandra$ observations from 2007 to
2019, we distinguish the `non-flaring state' from `flaring state' in the light
variability. After removing flaring state data, we identify 4 gas clumps in the
nucleus and all of them can be well fitted by the clumpy-ADAF model. The
average mass accretion rate is $\sim 0.16 \rm M_{\odot} yr^{-1}$. We analyze
the photon index($\Gamma$)-flux(2-10keV) correlation between the non-flaring
state and flaring state. For the non-flaring states, the flux is inversely
proportional to the photon index. For the flaring states, we find no obvious
correlation between the two parameters. In addition, we find that the flare
always occurs at a high mass accretion rate, and after the luminosity of the
flare reaches the peak, it will be accompanied by a sudden decrease in
luminosity. Our results can be explained as that the energy released by
magnetic reconnection destroys the structure of the accretion disc, thus the
luminosity decreases rapidly and returns to normal levels thereafter.
