The invention discloses a lost circulation additive for blocking loss of a shallow casing. The los circulation additive is prepared from the following components in parts by weight: 0.1-5 parts of rapid filtration fiber, 0.5-5 parts of bentonite, 0.5-3 parts of filter aid, 0.5-3 parts of enhancer and 0.5-2 parts of delayed expansion plugging agent. The lost circulation additive has high dehydration characteristics, and can effectively resist dilution of formation water. A main component natural plant fiber has strong hydrophily and large specific surface area, so that the formed plugged zone has stronger adhesive strength on the water-wetted rock surface, the plugged zone can be effectively prevented from being damaged due to flow-back of a leakage layer, delayed expansion particles in the lost circulation additive can absorb water to expand after a solid-phase plugged zone is formed, the strength of the plugged zone is further enhanced, and an effective barrier is built for subsequent cement cementing.