  Fluctuations of conserved quantities are sensitive observables to probe the
signature of QCD phase transition and critical point in heavy-ion collisions.
With the UrQMD model, we have studied the centrality and energy dependence of
various order cumulants and cumulant ratios (up to fourth order) of
net-proton,net-charge and net-kaon multiplicity distributions in Au+Au
collisions at $\sqrt{s_{NN}}$= 7.7, 11.5, 19.6, 27, 39, 62.4, 200 GeV. The
model results show that the production mechanism of the particles and
anti-particles have significant impacts on the cumulants of net-particles
multiplicity distributions and show strong energy dependence. We also made
comparisons between model calculations and experimental data measured in the
first phase of the beam energy scan (BES) program by the STAR experiment at
RHIC. The comparisons indicate that the baryon conservation effect strongly
suppress the cumulants of net-proton distributions at low energies and the
non-monotonic energy dependence for the net-proton {\KV} at the most central
Au+Au collisions measured by the STAR experiment can not be described by the
UrQMD model. Since there has no physics of QCD phase transition and QCD
critical point implemented in the UrQMD, the model results provide us baselines
and qualitative estimates about the non-critical background contributions to
the fluctuations observables in heavy-ion collisions.
