Due to their distinctive structure, inherently anisotropic properties and broad applications, Janus colloidal particles have attracted tremendous attention and it is significant to synthesize high yield Janus colloidal particles in a cost-effective and reliable way. On the other hand, due to the expanded electromagnetic interference problems, it is highly desired to develop excellent electromagnetic wave absorbing materials with an ultra-wide absorption bandwidth for practical application. Herein, a confined liquid–solid redox reaction strategy has been developed to fabricate a series of Fe_x(Co_yNi_1−y)_100−x ternary alloy particles. The as-prepared particles are in the form of u-channelled noncentrosymmetric spheres, one kind of Janus colloidal particles which have been rarely observed. Due to the combination and synergy effects of multi-magnetic metals, the polycrystalline structure and their specific morphology, the as-prepared particles possess multiple magnetic resonance and multiple dielectric relaxation processes, and therefore show excellent electromagnetic wave absorption performances. In particular, the strongest reflection loss (RL) of the Fe₁₅(Co_0.2Ni_0.8)₈₅ Janus colloidal particles is up to −36.9 dB with a thickness of 2.5 mm, and the effective absorption (RL < −10 dB) bandwidth can reach 9.2 GHz (8–17.2 GHz) with a thickness of 2 mm. Such a wide bandwidth has barely been reported for magnetic metal alloys under a single thickness. These results suggest that the Fe_x(Co_yNi_1−y)_100−x Janus particles could be a promising candidate for highly efficient electromagnetic wave absorbing materials for practical application.