In this study, the proton is taken as an ensemble of quark-gluon Fock states.Using the principle of detailed balance, the probabilities of finding every Fock states of the proton are obtained without any parameter. A new origin of the light flavor sea quark asymmetry, i.e.,ū =d, is given as a pure statistical effect. It is found thatd −ū ≈ 0.124, which is in surprisingly agreement with the experimental observation.