  We revisit constraints on annihilating dark matter based on the global 21cm
signature observed by EDGES. For this purpose, we used the numerical data of
the latest N-body simulation performed by state-of-art standard in order to
estimate the boost factor at high redshifts ($z$ = 10 - 100), which enhances
the annihilation of dark matter in course of structure formation. By taking
into account to what fraction injected energy from dark matter annihilation
contributes to ionization, excitation and heating of intergalactic medium
during dark ages, we estimated how large the global 21cm absorption can be. By
assuming the thermal freezeout scenario, we find that $m_{\rm DM} < 15$ GeV and
$m_{\rm DM} < 3$ GeV have been excluded at 95$\%$ C.L. for the annihilation
modes into $b\bar{b}$ and $e^+ e^-$, respectively.
