  We study the dilution of dark matter (DM) relic density caused by the
electroweak first-order phase transition (FOPT) in the singlet extension
models, including the singlet extension of the standard model (xSM), of the
two-Higgs-doublet model (2HDM+S) and the next-to-minimal supersymmetric
standard model (NMSSM). We find that in these models the entropy released by
the strong electroweak FOPT can dilute the DM density to 1/3 at most.
Nevertheless, in the xSM and NMSSM where the singlet field configure is
relevant to the phase transition temperature, the strong FOPT always happens
before the DM freeze-out, making the dilution effect negligible for the current
DM density. On the other hand, in the 2HDM+S where the DM freeze-out
temperature is independent of FOPT, the dilution may salvage some parameter
space excluded by excessive DM relic density or by DM direct detections.
