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Accretion of tidally disrupted stars (TDSs) is expected to contributesignificantly to the growth of massive black holes (MBHs) with mass $\sim 10^6M_{\odot}$ in galactic centers. In this paper, we quantitatively investigatethe effect of the TDS accretion on the spin evolution of these relatively smallMBHs, by also considering the accretion of gas-clouds with (many) chaoticepisodes. We find that the accretion of TDSs can play an important role or evena dominant role in shaping the spin distribution of $\sim 10^6 M_{\odot}$ MBHs,depending on the contribution fraction ($f_{\rm TDE}$) of the TDS accretion tothe MBH growth. If $f_{\rm TDE}$ is as large as $\gtrsim 0.9$, most $\sim10^6M_{\odot}$ MBHs have low spins ($|a| \lesssim 0.3$); if $f_{\rm TDE}$ is assmall as $\lesssim 0.1$, most $\sim 10^6 M_{\odot}$ MBHs have high spins ($|a|\gtrsim 0.7$). We also find that (1) the fraction of highly spinning $\sim 10^6M_{\odot}$ MBHs in the TDS accretion states is smaller than that in thegas-cloud (AGN) accretion states, which is a consequence of more rapid spindecrease during the period of consecutive TDS accretion than the spin increaseduring the AGN periods when the spin is large; (2) the fraction of retrogradespin accretion in the TDS accretion states is almost the same as that ofprograde spin accretion, while it is negligible in the gas-cloud (AGN)accretion states. Current scarce sample of AGNs ($\sim 10^6 M_{\odot}$) withspin measurements hints an insignificant contribution from TDS accretion to MBHgrowth. Future measurements on spins of $\sim 10^6 M_{\odot}$ MBHs may providestronger constraints on the importance of both AGN and TDS accretion states intheir growth history.

Effect of accreting tidally disrupted stars on the spin evolution of $\sim 10^6M_{\odot}$ black holes