Basing on first principls, we investigate the electrical properties of a molecular junction consisting of 2-amino-5-nitro-1,4-diethyny-4’-benzenethiol-benzene molecule and gold surface. Density functional theory is employed to obtain the electronic structures of the molecule and the extended molecule. Then we determine the interaction energy between the molecule and the gold surface quantitatively. The elastic Green function method is applied to study its current-voltage properties. Numerical results show that when the external applied bias is lower than 0.9V, there is a current gap. With increasing bias, the conductance of the junction exhibits plateaus. These electrical properties are closely related to the electronic structures of the molecular junction. The extended molecular orbits have great contribution to the charge transport, while localized molecular orbits contributes but little to the current where charge transport takes place by tunneling.