In this paper, a low profile horizontally polarized wideband omnidirectional antenna is presented, which consists of a simple single-layer dielectric substrate planar printing structure. The bottom of the substrate is loaded with six Vivaldi slits to achieve omnidirectional radiation. An equal-amplitude 1:6 power-division network is printed on the top of the substrate to provide a uniform feed. In addition, rectangular slots etched on each radiation element conduce to the enhancement of high-frequency gain and improvement of impedance matching. The antenna has 58.8% impedance bandwidth (2.8-5.13 GHz, VSWR<2) and low profile height of 0.009λmin, and it is convenient to fix under the ceiling of buildings and could radiate well in indoor place. The radiation mode in the whole operating frequency band is stable, and the cross-polarization is less than -20 dB, which completely covers the 5G NR-n77/78/79 band.