  The difference between vacuum energy of quantum fields in Minkowski space and
in Friedmann-Robterson-Walker universe might be related to the observed dark
energy. The vacuum energy of the Veneziano ghost field introduced to solve the
$U(1)_A$ problem in QCD is of the form, $ H+ {\cal O}(H^2)$. Based on this, we
study the dynamical evolution of a phenomenological dark energy model whose
energy density is of the form $\alpha H+\beta H^2$. In this model, the universe
approaches to a de Sitter phase at late times. We fit the model with current
observational data including SnIa, BAO, CMB, BBN, Hubble parameter and growth
rate of matter perturbation. It shows that the universe begins to accelerate at
redshift $z\sim 0.75$ and this model is consistent with current data. In
particular, this model fits the data of growth factor well as the $\Lambda CDM$
model.
