Within the framework of the parton picture which is derivable covariantly from the rest-frame hadronic wave function, the proton structure function is calculated from the Lorentz-deformed proton wave function and is compared with experiment. It is shown first that the hadronic momentum is not entirely carried by the valence quarks even in the large x region. A simple model is constructed to redistribute the parton momentum. This model can explain accurately the proton structure function, the Drell-Yan West relation and the neutron-proton structure function ratio, in the large x region. This model also gives a proton-neutron structure function difference for all values of x, which is in reasonable agreement with the experimental data. The antiquark contribution to the structure function can also be calculated.