  With the framework of UrQMD model, by tracing the number of initial quarks in
protons, we study the elliptic flow of transported protons, poduced protons and
anti-protons in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 39, 200 GeV.
The difference of elliptic flow between transported protons and anti-protons is
observed. The results of UrQMD model show a good agreement with the STAR
results at low energies, but are deviate at high energies. It partly explains
the $v_{2}$ difference between proton and anti-proton observed in the Beam
Energy Scan program in Relativistic Heavy Ion Collider~(RHIC).
