The invention discloses a steel frame based on mechanical full-face excavation of a tunnel, and a construction method thereof. The steel frame comprises an arch part grid steel frame unit and side part grid steel frame units located at two ends of the arch part grid steel frame unit, wherein the arch part grid steel frame unit and the side part grid steel frame units are hinged to be prefabricatedinto a whole structure in a factory. From the angle of force loading, according to the steel frame provided by the invention, the number of the steel frame units and connecting joints is greatly reduced, the weakening on the overall force loading strength of the steel frame is reduced maximally, the supporting intensity of the steel frame is ensured, the steel frame forms a whole, the positioningdifficulty is low and the efficiency is high after lifting, the steel frame can be directly lifted as a whole through a multi-arm steel frame installation trolley, mechanical operation is realized, the number of construction operation personnel is reduced, the construction safety is improved, a working environment for the construction personnel is favorable, the human cost and the labor intensityare saved, the operating time of a single process can be greatly reduced, the operation progress of tunnel initial supporting construction is accelerated as a whole, the economic benefit is remarkable, and the construction cost is saved.