  A broad excess of keV electron recoil events was reported by XENON1T
recently. This excess may be induced by the scattering of electrons and
long-lived MeV particles with high-speed. The properties of the new long-lived
particles are restricted by the big bang nucleosynthesis (BBN) and cosmic
microwave background (CMB). Here we consider a tangible model of two scalar MeV
dark matter (DM) particles $S_A$ and $S_B$ to interpret the XENON1T keV excess
via boosted $S_B$. A small mass splitting $m_{S_A}-m_{S_B}>0$ is introduced and
pairs of boosted $S_B S_B^\ast$ can be produced by the dark annihilation
process of $S_A S_A^\ast\to \phi \to S_B S_B^\ast$ via a resonant scalar
$\phi$. The $S_B-$electron scattering is intermediated by a vector boson $X$
which may cause the anomalous transitions of $^8$Be and $^4$He. Although
constraints from BBN, CMB and low-energy experiments set the $X-$mediated
$S_B-$electron scattering cross section to be $\lesssim 10^{-35}
\mathrm{cm}^2$, the MeV scale DM with an enhanced dark annihilation today can
still provide enough boosted $S_B S_B^\ast$ and induce the XENON1T keV excess.
The relic density of $S_B S_B^\ast$ is significantly reduced by the $s-$wave
process of $S_B S_B^\ast \to X X$ which is allowed by constraints from CMB and
21-cm absorption. A very small relic fraction of $S_B S_B^\ast$ is compatible
with the stringent bounds on un-boosted $S_B$-electron scattering in DM direct
detection and the un-boosted $S_A$-electron scattering is also allowed by the
direct detection.
