A low-frequency quarter-wave superconducting ß=1 structure has been proposed as a main accelerating cavity for the newly planned storage ring light source with ultra-low emittance. In the development of High Energy Photon Source, a 6 GeV diffraction-limited light source at the Institute of High Energy Physics, a compact 166.6 MHz proof-of-principle cavity was designed. By optimizing its geometry, the cavity’s mechanical properties have been significantly improved with a focus on stress, pressure sensitivity, Lorentz-force detuning, microphonics and tuning capabilities. The cavity was later fabricated, post-processed and tested in a vertical manner at cryogenic temperatures. High peak electromagnetic fields have been achieved with an excellent cryogenic performance, largely exceeding the design goal and project specifications. In addition, the measured mechanical parameters are in good accordance with simulation results. Small Lorentz coefficients and low pressure sensitivities were measured during the tests. This constitutes the first demonstration of a solid low-frequency ß=1 superconducting cavity design with compact geometry and favorable mechanical properties.