The invention discloses a plant multifunctional nutrient agent as well as a preparation method thereof and application thereof. The nutrient agent is prepared from the following raw materials in parts by weight: 13-21 parts of ferrous sulfate, 15-25 parts of chitosan oligosaccharide, 7-15 parts of triacontanol, 11-19 parts of potassum fulvic acid, 1-5 parts of catechinic acid, 0.5-2 parts of allicin and 80-100 parts of water. The plant multifunctional nutrient can promote plant growth, enhances plant immunity, effectively prevents plant diseases and insect pests, enables the plant to have various ecological regulation functions of stress resistance, cold resistance and drought resistance, keeps softness and viscidity of soil application, keeps biological diversity of soil, improves degraded and hardened soil, is beneficial for plant growth, promotes fruit expansion, shortens the production period, can guarantee the quality and increases the yield, has very good plant protection effect and very good growth promotion effect, and relieves damages caused by weather, pesticide hazards, fertilizer damages, hormone intoxication and plant diseases and insect pests; and moreover, product performances are stable, the quality guarantee period is long, and large-scale industrial production is adapted.