Capsule model of gold nanorods is put forward and absorption spectra are calculated using T-matrix method developed by Waterman. Experimental spectra and calculated spectra agree well. The absorption maximum at around 520 nm is corresponding to the transverse surface plasmon resonance (transverse mode), while the absorption maximum at longer wavelength is corresponding to the longitudinal surface plasmon resonance (longitudinal mode). The absorption maximum of longitudinal mode red-shifts significantly with increasing aspect ratio of gold nanorods, while the absorption maximum of transverse mode blue-shifts slightly. Besides, calculations show that the medium dielectric constants decrease nonlinearly with increasing aspect ratio of gold nanorods.