The tight-binding method is employed to investigate the effects of three typical in-plane electric fields on the electronic structure of a triangular zigzag graphene quantum dot. The calculation shows that the single-electron eigenstates evolute independently in two subspaces no matter how the electric fields change. The electric field with fixed-geometry gates chooses several scattered parts of the zero-energy eigenspace as the new zero-energy eigenstates, regardless of the field strength. Moreover, the new zero-energy eigenstates remain unchanged and the associated levels are linear with the field strength. In contrast, the new nonzero-energy eigenstates mix mutually and the associated levels are nonlinear with the field strength. By comparing the effects of three electric fields, we demonstrate that the degeneracy of the zero-energy eigenstates accounts for the linearity of the associated levels.