β-ammonium tetramolybdate [β-(NH₄)₂Mo₄O₁₃] was prepared in a concentric structure reactor. The species and microstructure of Mo with the change of solution acidity in the preparation process were investigated. The results showed that the pH of the solution should be controlled below 2.5 to obtain ammonium tetramolybdate crystals. In the acidification process, NH₃？H₂O was used to adjust the pH back to about 3.0 to dissolve the fine crystals produced and facilitate the conversion of ammonium heptadmolybdate to ammonium tetramolybdate. Decreasing the soaking time between the (NH₄)₄Mo₄O₁₂(O₂)₂？2H₂O crystals and the original liquor was beneficial to obtaining β-(NH₄)₂Mo₄O₁₃. Based on the polymerization mechanism of molybdenum, a multi-lattice concentric circle reactor was designed to realize the regional regulation of ammonium molybdate crystallization. High-quality β-(NH₄)₂Mo₄O₁₃ was prepared continually by periodically adding HNO₃ solution, ammonia water, and product slurry under specified conditions.