Petroleum pipelines across slopes can produce obvious mechanical responses to seismic landslide thrust. In order to rationally calculate internal forces and displacements of the pipeline due to the seismic landslide of the soil mass surrounding it, a mechanical analysis method of the pipeline is proposed based on the Timoshenko beam considering lateral shear deformation on the Winkler foundation with the normal trapezial earth pressure on it, in which the seismic earth pressure is determined using the transfer coefficient method combined with the pseudo-static method. The formulas of internal forces and displacement of the pipeline are accordingly derived, and the material determination approach is put forward according to the control of its safe stress state. Analysis results of an example show that the pipeline deflection increases with the distance to the slide mass boundary, and the maximum bending moment and shear force on the pipeline are nearly located in the boundary, which is consistent with that by the numerical simulation via FLAC3D. The distribution mode of the internal forces is not almost varied with seismic coefficients, pipeline length in the slide mass, and cross-section of the pipeline. However, the maximum bending moment and shear force are nonlinearly increasing with the horizontal seismic coefficient, slightly increasing with the cross-section, and marginally decreasing with the increase of the pipeline length in the slide mass.