  How the environment influences the most massive galaxies is still unclear. To
explore the environmental effects on morphology and star formation in the most
massive galaxies at high redshift, we select galaxies with stellar mass
$\log(M_{\star}/M_{\odot})>11$ at $0.5<z<2.5$ in the COSMOS-DASH field, which
is the largest field with near-infrared photometrical observations using
HST/WFC3 to date. Combining with the newly published COSMOS2020 catalog, we
estimate the localized galaxy overdensity using a density estimator within the
Bayesian probability framework. With the overdensity map, no significant
environmental dependence is found in the distributions of S\'{e}rsic index and
effective radius. When we consider the star formation state, galaxies in lower
density are found to have higher median specific star formation rate (sSFR) at
$0.5<z<1.5$. But for star-forming galaxies only, sSFR is independent of the
environment within the whole redshift range, indicating that the primary effect
of the environment might be to control the quiescent fraction. Based on these
observations, the possible environmental quenching process for these massive
galaxies might be mergers.
