The invention provides a gravel mixing system and method for an underground cavern. The gravel mixing system comprises a movable trolley. A gravel aggregate production device, a conveying device and a mixing device are arranged on the movable trolley. A construction method comprises the following steps that cavern residues are charged into a feeding bin and are roughly broken through a roughly-breaking workshop and finely machined into aggregates through a fine crushing workshop, the aggregates are conveyed into a screening workshop, screening and selection are conducted, and during concrete production, a mixed material storage bin conveys the mixed aggregates to a mixing station through the conveying device for production; or when the sprayed and mixed concrete aggregates are required, a sprayed and mixed concrete storage bin conveys the aggregates to the mixing station for production; when a certain length of the lining support is completed, the whole movable trolley is moved, and the whole trolley is moved to a new preset position by the adoption of traction equipment or an automatic drive device; and by means of the steps mentioned above, synchronous moving of the underground cavern gravel mixing and lining support is achieved. The gravel mixing system and method for the underground cavern can conveniently operate along with the construction of the lining support, the purchase cost of raw materials is further saved, and the transportation distance is shortened.