We investigate the parton distribution function (PDF) uncertainty in the measurement of the effective weak mixing angle at the CERN Large Hadron Collider (LHC). The PDF-induced uncertainty is large in proton-proton collisions at the LHC due to the dilution effect. The measurement of the Drell-Yan forward-backward asymmetry ( ) at the LHC can be used to reduce the PDF uncertainty in the measurement. However, when including the full mass range of lepton pairs in the data analysis, the correlation between the PDF updating procedure and the extraction leads to a sizable bias in the obtained value. From our studies, we find that the bias can be significantly reduced by removing Drell-Yan events with invariant mass around the Z-pole region, while most of the sensitivity in reducing the PDF uncertainty remains. Furthermore, the lepton charge asymmetry in the W boson events as a function of the rapidity of the charged leptons, , is known to be another observable which can be used to reduce the PDF uncertainty in the measurement. The constraint from is complementary to that from , and thus no bias affects the extraction. The studies are performed using the error PDF Updating Method Package (ePump), which is based on Hessian updating methods. In this article, the CT14HERA2 PDF set is used as an example.