Abstract: In order to solve the problem of low strength and brittleness of coastal soft soil, this paper studies the composite improvement effect of lime-fly ash and PVA fiber. An orthogonal experimental design evaluated the effects of lime-fly ash content and fiber length on unconfined compressive strength (UCS) and splitting tensile strength (STS). Microstructural analysis via scanning electron microscopy (SEM) elucidated reinforcement and toughening mechanisms. Results indicate: (1) The lime-fiy ash content dominates strength development; UCS and STS increase approximately 3-5 times as content rises from 5% to 30%. Fiber length primarily governs toughness, with 6 mm fibers exhibiting optimal mechanical performance due to effective anchoring. (2) The addition of PVA fiber shifts the failure mode from brittle splitting to ductile plastic failure, significantly retarding post-peak strength degradation. (3) Pozzolanic reaction products (C-S-H/C-A-H) fill the pores and construct a dense matrix, while hydration gel on fiber surfaces enhances fiber-soil bonding via mechanical interlocking and friction. Progressive fiber pull-out during loading serves as the primary energy dissipation mechanism, thereby improving macroscopic toughness.