  Recently the Experiment to Detect the Global Epoch of Reionization Signature
(EDGES) reported the detection of a 21cm absorption signal stronger than
astrophysical expectations. In this paper we study the impact of radiation from
dark matter (DM) decay and primordial black holes (PBH) on the 21cm radiation
temperature in the reionization epoch, and impose a constraint on the decaying
dark matter and PBH energy injection in the intergalactic medium, which can
heat up neutral hydrogen gas and weaken the 21cm absorption signal. We consider
decay channels DM$\rightarrow e^+e^-, \gamma\gamma$, $\mu^+\mu^-$, $b\bar{b}$
and the $10^{15-17}$g mass range for primordial black holes, and require the
heating of the neutral hydrogen does not negate the 21cm absorption signal. For
$e^+e^-$, $\gamma\gamma$ final states and PBH cases we find strong 21cm bounds
that can be more stringent than the current extragalactic diffuse photon
bounds. For the DM$\rightarrow e^+e^-$ channel, the lifetime bound is
$\tau_{\rm DM}> 10^{27}$s for sub-GeV dark matter. The bound is $\tau_{\rm
DM}\ge 10^{26}$s for sub-GeV DM$\rightarrow \gamma\gamma$ channel and reaches
$10^{27}$s at MeV DM mass. For $b\bar{b}$ and $\mu^+\mu^-$ cases, the 21 cm
constraint is better than all the existing constraints for $m_{\rm DM}<20$ GeV
where the bound on $\tau_{\rm DM}\ge10^{26}$s. For both DM decay and primordial
black hole cases, the 21cm bounds significantly improve over the CMB damping
limits from Planck data.
