In situ plugging hydrogels represent a promising strategy to combat wellbore instability in fractured formations. Despite their potential, they often fail due to unpredictable gelation kinetics and inadequate mechanical strength under downhole conditions. Here, we introduce an alginate-based hydrogel (Alg-gel) engineered with an acid-triggered, multi-crosslinking mechanism that constructs a biodegradable shield directly within fractures. This system integrates sodium alginate (SA) with hydroxypropyl guar gum (Hpg) to form a primary semi-interpenetrating network. The critical innovation lies in the synergistic use of D-gluconic acid δ-lactone (GDL) and CaCO₃, which enables precise, sustained release of Ca²⁺ ions. These ions subsequently coordinate with guluronate blocks in SA, establishing a secondary network that embeds residual CaCO₃ as reinforcing scaffolds. This multi-network architecture results in a storage modulus increase by orders of magnitude and reduces filtration loss by up to 89.3% as gelation proceeds from 30 to 180 min. Structural evolution from a sparse framework to a densely interlocked lamellar assembly was directly visualized, validating the tunable nature of the complexation process. The exceptional plugging performance and controllable gelation kinetics position Alg-gel as a superior lost circulation material, with broader implications for profile modification and gas channeling mitigation.