The invention relates to a seawater-based instant dissolving fracturing fluid and a preparation method thereof. The seawater-based instant dissolving fracturing fluid comprises the following components by parts: 65-99.99 parts of a base fluid and 0.01-35.0 parts of a cross-linked fluid, wherein the base fluid comprises 95.0-99.6 parts of water, 0.1-1.0 part of a gelling agent, 0.1-2.0 parts of a colloid protective agent, 0.1-1.0 part of a synergistic agent and 0.1-1.0 part of an emulsion resolving cleanup additive; the cross-linked fluid comprises 0-99.9 parts of water, 0.1-100.0 parts of a cross-linking agent and 0.001-0.5 part of a gel breaker. The seawater-based instant fracturing fluid, prepared according to the invention has the properties of instant dissolving, salt resisting and high temperature resisting, can meet the mode of a mixing vehicle self-absorbing seawater as well as preparing and fracturing the fracturing fluid at the same time, the cost and the risk of the construction are reduced, and large-scale application is promoted; comparing with the conventional mode, the composite cost is reduced by 30% above, the fracturing mode on the sea is innovated, and the seawater-based instant dissolving fracturing fluid has a good economic benefit and an enormous social benefit, and has a wide application prospect.