In this paper, a theoretical model describing the surface superfluidity in 4He-3He mixture films is proposed to obtain an expression for the potential energy of vortex pairs in the presence of 3He. The static recursion relations as well as the dynamical delectric function are then derived using this form of potential energy. The period shift 2△P/P and the dissipation Q-1 for the torsional oscillator near TC are calculated theoretically and compared with the experimental results of McQueeney et al.