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This article discusses the methods to stabilize potentially liquefied soils in Yogyakarta International Airport (YIA) area. It presents an innovative bio-cementing soil improvement method to increase the shear strength of loose sand in soil conditions with initial salinity content 3.4%. The type of bacteria chosen was Bacillus sp, which can survive in salinity and aerobic conditions and precipitate microbial-induced calcium carbonate (MICP). This improvement was realized by mixing loose sand with fly ash as a source of Calcium (Ca) and microbial fluids. The aim of this article is to investigate the performance and mechanism of MICP in the binding process between loose sand particles by optimizing the percentage of fly ash and microbial in salinity conditions. During the MICP process, microbes require Ca2+ ions obtained from fly ash, which contains SiO2 and CaO to generate CaCO3 for the bonding process between particles. The percentage of fly ash used, loose sand moisture content, and curing time were examined to identify an increase in the shear strength value of the sample. The results showed that the shear strength of the samples treated with the addition of microbial fluid and a 35% fly ash mixture under salinity conditions exhibited a significant increase compared to other combinations. This indicated that the addition of 35% fly ash and 0.2 mL of microbial liquid can result in a shear strength of 78.08ᵒ, cohesion of 0.18 gr/cm3, and an increase in grain dimensions due to bonding between particles as observed in the results of the sieve analysis after a curing time of 28 days. Furthermore, the MICP mechanism, including the interaction of bacteria, fly ash, and sand in binding soil particles with the production of CaCO3, was observed using a digital microscope and scanning electron microscope based on the results of XRD and SEM. [ABSTRACT FROM AUTHOR]