  We present our photometric and spectroscopic observations on the peculiar
transient AT2018cow. The multi-band photometry covers from peak to $\sim$70
days and the spectroscopy ranges from 5 to $\sim$50 days. The rapid rise
($t_{\mathrm{r}}$$\lesssim$2.9 days), high luminosity
($M_{V,\mathrm{peak}}\sim-$20.8 mag) and fast decline after peak make AT2018cow
stand out of any other optical transients. While we find that its light curves
show high resemblance to those of type Ibn supernovae. Moreover, the spectral
energy distribution remains high temperature of $\sim$14,000 K after $\sim$15
days since discovery. The spectra are featureless in the first 10 days, while
some broad emission lines due to H, He, C and O emerge later, with velocity
declining from $\sim$14,000 km s$^{-1}$ to $\sim$3000 km s$^{-1}$ at the end of
our observations. Narrow and weak He I emission lines emerge in the spectra at
$t>$20 days since discovery. These emission lines are reminiscent of the
features seen in interacting supernovae like type Ibn and IIn subclasses. We
fit the bolometric light curves with a model of circumstellar interaction (CSI)
and radioactive decay (RD) of \Ni and find a good fit with ejecta mass
$M_{\mathrm{ej}}\sim$3.16 M$_{\odot}$, circumstellar material mass
$M_{\mathrm{CSM}}\sim$0.04 M$_{\odot}$, and ejected \Ni mass
$M_{^{56}\mathrm{Ni}}\sim$0.23 M$_{\odot}$. The CSM shell might be formed in an
eruptive mass ejection of the progenitor star. Furthermore, host environment of
AT2018cow implies connection of AT2018cow with massive stars. Combining
observational properties and the light curve fitting results, we conclude that
AT2018cow might be a peculiar interacting supernova originated from a massive
star.
