We present our photometric and spectroscopic observations of the peculiar transient AT2018cow. The multiband photometry covers from peak to ~70days, and the spectroscopy ranges from 5 to ~50days. The rapid rise (t_r_<~2.9days), high luminosity (M_V,peak_~-20.8mag), and fast decline after peak make AT2018cow stand out from any other optical transients, whereas we find that its light curves show a high resemblance to those of Type Ibn supernovae. Moreover, the spectral energy distribution remains at a high temperature of ~14000K at t>15days after discovery. The spectra are featureless in the first 10days, while some broad emission lines due to H, He, C, and O emerge later, with velocity declining from ~14000 to ~3000km/s at the end of our observations. Narrow and weak He I emission lines emerge in the spectra at t>20days after discovery. These emission lines are reminiscent of the features seen in interacting supernovae like the Type Ibn and IIn subclasses. We fit the bolometric light curves with a model of circumstellar interaction and radioactive decay of ^56^Ni and find a good fit with ejecta mass M_ej_~3.16M_{sun}_, circumstellar medium (CSM) mass M_CSM_~0.04M_{sun}_, and ejected ^56^Ni mass M_56Ni_~0.23M_{sun}_. The CSM shell might be formed in an eruptive mass ejection of the progenitor star. Furthermore, the host environment of AT2018cow implies a 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 that originated from a massive star.