A kind of three-dimensional local resonance phononic crystal structure was proposed. By FEM calculation, low frequency bandgap characteristic, multiple vibration coupling mechanism and corresponding influence factors of geometric parameters were analyzed. Results show that this kind of structure can open ultra-low frequency bandgap under 50 Hz and critical factor is the vibration coupling effect between the matrix material and cylindrical harmonic oscillator. The more vibration displacement of lower surface on cylindrical harmonic oscillator is, the wider bandgap is. Density of middle oblique bar has no effect on the lower edge of bandgap, but makes the upper edge of bandgap move to the higher frequency range, and hence results in the change of bandgap. Length of middle oblique S₂ section and angle of S₁ section are the most important factor in bandgap. This study enriches the design and the equivalent model of three-dimensional phononic crystal frequency structure, which possesses a certain guiding value in engineering application.