The invention relates to composite fiber reinforced foam concrete and a preparation method thereof. The foam concrete comprises the following components in parts by weight: 0-400 parts of fly ash, 100-900 parts of cement, 500-1000 parts of sands, 300-600 parts of water, 0.4-1 part of organic fiber, 1-3 parts of plant fiber, 0.2-15 parts of early strength agent, 3-20 parts of foaming agent, 0.5-4 parts of water reducing agent and 0.5-2 parts of water retention agent. The preparation method of the foam concrete comprises the following steps of: generating foams by using the foaming agent, adding the foams to fully stirred cement mortar containing composite fibers and the like and keeping stirring so that the foams are dispersed in the mortar uniformly, measuring the volume weight and carrying out casting forming or coating forming and carrying out standard curing for 28 days. In the invention, industrial and agricultural wastes, such as fly ash, plant fiber and the like, are used, shrinkage crack of the foam concrete is inhibited obviously, and the composite fiber reinforced foam concrete has low price; and the foam concrete has simple formula, good workability, long foam stabilizing time, light weight and low coefficient of thermal conductivity and is suitable for exterior wall thermal insulation, roof thermal insulation and non-load-bearing wall manufacturing.