Insect frass of black soldier flies fed with kitchen waste generally exhibits a relatively high salt content, which poses certain constraints on its utilization as organic fertilizer. This study was designed to investigate the impacts of fermentation with salt-tolerant growth-promoting bacteria on the composition of the insect frass and its application efficacy, thus offering a reference for alleviating the adverse effects associated with the application of black soldier fly frass. Four salt-tolerant strains isolated from halophytes were formulated into a mixed bacterial suspension. This suspension was inoculated into the black soldier fly frass at an inoculation ratio of 10% (v/w) and subjected to aerobic fermentation at 30℃ for 28 days. After fermentation, the composition of the insect frass was determined. The fermented (F) and unfermented (UF) insect frass were separately added to the horticultural substrate at proportions of 2.5%, 5%, 10%, and 20% (v/v). They were then thoroughly blended and filled into aperture disk. Chinese cabbage ‘Suzhou Green’ was sown and cultivated at room temperature for 30 days. The treatment without the addition of insect frass was designated as the control (CK). After the cultivation phase, the plant indices and the physico-chemical properties of the substrate were measured. The results revealed that, in terms of the insect frass composition, fermentation with salt-tolerant growth-promoting bacteria significantly enhanced the organic matter and total nutrient content of the black soldier fly frass (P<0.05). Nevertheless, it led to a notable decrease in humic acid and available nutrients (P<0.05). When the fermentation duration was within 21 days, the content of water-soluble salts in the fermented insect frass also decreased to a certain extent. Regarding the influence on Chinese cabbage, when the insect frass addition level was 2.5% and 5%, the root indices, leaf area and average dry weight of Chinese cabbage in the unfermented insect frass treatment were higher compared to those in the fermented insect frass treatment. Conversely, when the addition level was 10% and 20%, the corresponding indices of Chinese cabbage in the unfermented insect frass treatment were lower than those in the fermented insect frass treatment. In terms of the impact on the substrate, when the insect frass addition amount was 2.5% and 5%, there was no significant disparity between the substrates with unfermented and fermented insect frass additions. When the addition amount was 10% and 20%, the available nutrients and EC value of the substrate with unfermented insect frass addition were significantly higher than those of the fermented insect frass addition (P<0.05). In summary, fermentation with salt-tolerant growth-promoting bacteria can, to a certain degree, improve the quality of black soldier fly frass sourced from kitchen waste and mitigate the negative impacts stemming from its application.