The interaction between a semiconducting single-walled carbon nanotube and its graphite substrate was studied by calculation based on the density functional theory. Between two contact configurations of the system energy minimum, a series of metastable contact configurations of the carbon nanotube on the graphite were optimized, and the atomic structures of these contact configurations and the related energy bands of the system were determined. It is found that the interaction makes the contacted graphite surface concave and the band gap of the carbon nanotube narrow, which may be significant to the carbon-based nanodevice strategy.