Abstract: Desiccation cracking in expansive soils often triggers geotechnical hazards. This study demonstrates that the incorporation of biochar and sisal fibers can effectively inhibit crack development. Through desiccation cracking tests,scanning electron microscopy(SEM),and digital image processing performed on compacted expansive soil from Nanning,the individual and combined effects of biochar and sisal fibers on crack resistance were systematically evaluated. The results show that untreated soil exhibited typical cracking patterns with interconnected primary cracks and secondary branching,while modified soil developed only primary cracks without secondary propagation. Moisture loss in modified soil followed an exponential decay model,where increased biochar content reduced the decay rate coefficient b,whereas higher fiber content and longer fiber length accelerated evaporation. Compared to untreated soil,the crack ratio decreased by 86.8% with 10% biochar and by 72.5% with 0.6% sisal fiber(by dry soil mass),respectively. In composite modifications,biochar content primarily governed crack length,while fiber content controlled crack ratio and width. Notably,a mixture of 10% biochar and 0.45% sisal fiber(20 mm length)completely suppressed surface cracking. Microstructural analysis revealed that biochar particles mechanically interlocked with soil through their rough surfaces and stored free water within internal pores,while randomly distributed sisal fibers formed a three-dimensional reinforcement network. Their synergistic interaction effectively restrained crack initiation and propagation.