Understanding the formation mechanism of giant molecular clusters is essential for rational design and synthesis of cluster-based nanomaterials with required morphologies and functionalities. Here, typical synthetic reactions of a 2.9 nm spherical molybdenum oxide cluster, {Mo-132} (formula: [(Mo72Mo60O372)-Mo-VI-O-V(CH3COO)(30)(H2O)(72)](42-)), with systematically varied reaction parameters have been fully explored to determine the morphologies and concentration of products, reduction of metal centers, and chemical environments of the organic ligands. The growth of these clusters shows a typical sigmoid curve, suggesting a general multistep self-assembly mechanism for the formation of giant molecular clusters. The reaction starts with a lag phase period when partial Mo-VI centers of molybdate precursors are reduced to form {Mo-2(V)(acetate)} structures under the coordination effect of the acetate groups. Once the concentration of {Mo-2(V)(acetate)} reaches a critical value, it triggers the co assembly of Mo-V and Mo-VI species into the giant clusters.