We report a simple approach to modulate the electron transport (ETp) band gap of protein (bovine serum albumin, BSA) by doping other molecule (theophylline, TPY). The doping was first confirmed by both the theoretical simulation (molecular docking) and experimental characterization (fluorescence spectra), which demonstrated the formation of the TPY–BSA complex with the TPY positioned inside a hydrophobic cavity at subdomain IIA of BSA. The TPY–BSA was then immobilized on an Au substrate surface with the same orientation via the –SH group of the Cys34 on the protein surface. The current-voltage (I V) results show that doping TPY can improve the ETp ability of BSA with its conduction band (CB) lowering and valence band (VB) raising, and thus achieve the purpose of modulating the protein ETp band gap. The results presented in this work may provide a general approach to regulate protein ETp band gaps with wide variability by preselecting the doping molecule. The doped protein may further serve as a suitable material for configuring nanoscale solid-state bioelectronic devices.