Abstract: The preparation of Fe₃O₄ nanoparticle by liquid-phase coprecipitation was introduced. And the orthogonal experiment was designed. The optimized condition that the proportion of Fe³⁺/Fe²⁺ was 1:2, crystallization temperature of Fe₃O₄ nanoparticle was 90℃ and crystallization time of Fe₃O₄ nanoparticle was 45min was achieved. Under the condition, the average size of nanoparticle is 10nm to 20nm. And BP nerve network is used to analyze the influence factors of the size of Fe₃O₄ nanoparticle such as proportion of Fe³⁺/Fe²⁺, crystallization temperature of Fe₃O₄ nanoparticle and crystallization time of Fe₃O₄ nanoparticle.