The invention belongs to the field of agricultural solid manure, and particularly relates to a water-retaining biological organic fertilizer used for mango and a preparation method thereof. According to the water-retaining biological organic fertilizer used for mango provided by the invention, mango bare branches, grass carbon, animal manure and municipal organic waste are used as raw materials, decayed materials are obtained through fermentation and decay of microorganism fungus, and then agricultural water retention agent is added in the decayed materials so as to obtain the water-retaining biological organic fertilizer used for mango, wherein the water-retaining biological organic fertilizer used for mango comprises the following raw materials according to parts by weight: 0-10 parts of mango bare branches, 25-35 parts of grass carbon, 30-40 parts of animal manure, 20-30 parts of municipal organic waste, 0.01-0.02 parts of microorganism fungus, and 0.1-0.4 parts of agricultural water retention agent. Through the utilization of the combination of mango bare branches, grass carbon, animal manure and municipal organic waste, special water-retaining biological organic fertilizer for mango for tip promoting, flower forcing and fruit enlarging is developed, so that waste is changed into valuable. Simultaneously, the water-retaining biological organic fertilizer used for mango obtained in the invention is good for water retaining, fertilizer maintenance and temperature preservation, and the structural performance of soil is improved; and the prepared particle biological organic fertilizer is resistant to storage and not easy for dehydration to cause particle scattering.