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The discharge mode transitions and the characteristics of atmospheric pressure dielectric barrier discharge in pure helium (He-DBD) are studied. Different discharge modes of He-DBD are clarified, including filament-diffuse mixed mode, filamentary mode, diffuse mode, and multichannels coupled mode. The transitions of different modes are explored by changing the gap width and applied voltage. According to the luminous structure of discharge [short exposure (100~300 ns) image], it is found that there should be only Townsend-like and glow-like discharges that appear in the above modes. In Townsend-like discharge, the secondary electron emission coefficient (SEEC) of dielectric is estimated. Based on which, a fluid model coupled with an external circuit is built to explain the He-DBD mode transitions mechanism and the applied voltage drop’s influence on He-DBD. Through the temporal evolution of a luminous structure, a discharge filament is believed to be a contractive state of discharge channel, which should be an unstable glow-like discharge. The radius of discharge channel, changing with gap width and applied voltage, is measured. The discharge contraction is attributed to the nonlinear relationship between Townsend ionization coefficient and electric field. The enhancing interaction between different discharge channels inhibits the appearing of diffuse discharge around discharge filament, leading to the transition from mixed mode to filamentary mode. In the end, the multichannels coupled mode is proven to be glow-like discharge, and its formation mechanism is explained by the overlap of discharge channels. [ABSTRACT FROM AUTHOR]