Soil aggregates support diverse microbes due to heterogeneous micro-environment. A lot of researches have exhibited the difference of microbial composition and activity within different size soil aggregates, but the relative influences of these microbes and the mechanisms underlying their effects on plant health are still poorly understood. This study investigated the microbiomes within four soil aggregate fractions sampled from fields with different incidences of tomato bacterial wilt derived from three fertilization regimes (organic, bio-organic and chemical) and un-fertilized soil to decipher the mechanisms involved in disease suppression. A wet-sieving method was used to separate the aggregate fractions; Illumina MiSeq sequencing was used to characterize the soil microbiomes in field experiment, and real-time qPCR analysis was used in lab cultivation experiment to quantify the number of pathogens. Organic fertilization (OF) and bio-organic fertilization (BF) significantly decreased disease incidences compared with the effects of treatments with chemical fertilizer (CF) and those without fertilizer (CK). The microbial composition was significantly different between fertilizations and aggregate fractions; particularly, the bacterial composition was significantly correlated with disease incidence. Different aggregate fractions contained disparate bacterial taxa correlated with disease incidence. Only in the microaggregate (Mi), the Ralstonia genus’ relative abundance showed a significant and positive correlation with disease incidence. The lab cultivation experiment demonstrated that after a spiking of Ralstonia solanacearum , whole soil and the Mi from BF-treated soil showed a significant higher resistance against pathogen invasion than those from CF-treated soil. The correlation between pathogen abundance and disease incidence in the field experiment and the higher resistance of Mi fraction against pathogen indicates that the microaggregates are the key fraction for suppressing tomato bacterial wilt in bio-organic fertilization practice, providing novel insight into the manipulation of the soil microbiome.