We study the physical mechanism of near-field interference of slit doublet and explore the distinctive phenomena in near-field interference of slit doublet. The average electric field and average energy-flow density are chosen to describe the near-field distribution rules of the electromagnetic field. The numerical calculations for near-field interference of slit doublet under different illumination conditions are performed according to the finite-difference time-domain method, and the distinctive characteristics of the near-field interference of slit doublet are provided. They include the polarization dependence, the distortion of fringes, and the bifurcation of the fringes. These characteristics are completely different from those of the far-field interference. With the aid of the interference of two slits and of slit and groove, the physical mechanism of polarization dependence of near-field interference is investigated. The fringe distortion of the electric field and the fringe bifurcation of the energy-flow density reflect the amplitude and phase variations of the electromagnetic field. The influences of the slit parameters and the base material on the near-field interference of slit doublet are also discussed. These results may provide us with new insights into the underlying physics of interaction between complex nanostructures and electromagnetic waves.