The surface plasmon self-interference excited by a strongly focused, linearly polarized vortex beam at off-axis illumination in a paraxial regime is analytically studied. The off-axis excitation is investigated using a geometrical model. The combination of an angular spectrum representation and homogeneous transformation is applied to derive the integral expressions of the surface plasmon polariton fields for off-axis directions both parallel and perpendicular to polarization plane, and an off-axis convergence angle is used to compute the integral. The surface plasmon excitation is represented by the relative peak intensity of the longitudinal field, while its standing wave is characterized by the full width at half-maximum of the transmitted field intensity distribution profile. Both models consistently show that even in ideal Gaussian microscopic imaging systems, self-interference degradation exists. When the off-axis angle increases, the surface plasmon interference disappears and the fields detune out of surface plasmon resonance.