  We study the decay of $B^+$ meson into $K^+$ plus a light mediator $\phi$
followed by production of dark matter (DM) pair $\bar \chi \chi$, which can
mimic the flavor changing neural current (FCNC) process of $B^+ \rightarrow K^+
\nu \bar \nu$ in the standard model (SM). Constraints from DM relic density,
direct detection, collider and $B-$physics data combined with the recently
reported $B^+\rightarrow K^+ \nu \bar \nu$ branching ratio performed at the
Belle II experiment, we analyse couplings between the mediator and the SM
fermions as well as the dark matter particle. We obtain that: if the process
$\phi \rightarrow \bar \chi \chi$ is kinematically allowed, i.e. $m_\phi >
2m_\chi$, then $m_\phi$ should be larger than mass of the muon pair; Otherwise,
$\phi$ should play the role of missing energy, and only allowed decaying
channel is $\phi \rightarrow e^+ e^-$.
