The invention relates to a compact ultra-wideband omnidirectional antenna comprising a first metal layer, a second metal layer and a dielectric substrate. The first metal layer and the second metal layer are printed on the upper layer and the lower layer of the dielectric substrate respectively, and each metal layer is composed of a double-branch flag-shaped radiator, double parasitic bands, a curved groove, a short open-circuit branch knot and a center metal disc. A short branch is loaded on a conical feed structure, the reactance characteristic of the antenna at high frequency is effectively suppressed, a high-frequency resonance point is introduced, and when double parasitic bands are added, a new resonance point is added between two existing resonance points, so that the bandwidth is improved, the 1.8-3.85 GHz passband characteristic is formed. A short open-circuit branch knot is added to the tail end of the first branch knot, a curve-shaped groove is etched, capacitive coupling and a current path are increased, and the low-frequency cut-off frequency is reduced. Finally, the impedance bandwidth of the antenna can reach 78% (1.69-3.85 GHz), in the impedance bandwidth, the cross polarization level on the horizontal plane is smaller than-20 dB, and the efficiency in the working frequency band is 80%-91%.