Understanding the microstructure, interaction and synergistic effect of building blocks in hybrid material can offer immense potential for the rational construction and applications of microwave absorbers. Herein, FeNi3 nanocrystals decorated on N-doped graphene nanosheets were fabricated as high-efficiency synergistic microwave absorbers. The morphologies of FeNi3 nanocrystals can be controlled between hollow/porous structure and clusters by changing the amount of N-doped graphene. Combined synergistic effect of magnetic FeNi3 nanocrystals and dielectric N-doped graphene, the hybrids exhibit greatly improved microwave absorption capacities compared to FeNi3 nanocrystals and N-doped graphene. The optimal hybrids show a maximum reflection value of -57.2 dB with an ultrathin thickness of only 1.45 mm, and the corresponding effective bandwidth is larger than 3.4 GHz (14.6-18 GHz). The remarkably enhanced microwave absorption properties of the hybrids are mainly originated from the effective impedance matching condition, improved attenuation capacity, well complementarities between magnetic loss from FeNi3 nanocrystals and dielectric loss from N-doped graphene, and multiple interfacial polarizations. These results suggest that the FeNi3/N-doped graphene hybrids with strong absorption, broad bandwidth, light weight and ultrathin thickness are promising candidate towards the commercialization of microwave absorbers.