In this paper, we apply the full new method of quantum theory to study the double-slit diffraction of C60 molecules. We calculate the double-slit wave functions of C60 molecules by Schrödinger equation, and calculate the diffraction wave function behind the slits with the Feynman path integral quantum theory, and then give the relation between the diffraction intensity of double-slit and diffraction pattern position. We compare the calculation results with two different double-slit diffraction experiments. When the decoherence effects are considered, the calculation results are in good agreement with the two experimental data.