Raman spectrum was applied to analyze the synthesis mechanism of nanostructure porous ZnO microspheres prepared via hydrothermal method assisted with trisodium citrate. The Raman spectrum characteristics of the sample revealed that the ZnO microspheres contained Zn-citrate complex, which was the complex of citrate acid group and Zn2+ in the reaction solution. The complex was chemisorbed on (204) and (503) faces of the Zn(OH)2 crystallite in the reacting solution, resulting in Zn(OH)2 nanosheet from the crystallite. Large quantities of Zn(OH)2 nanosheets aggregated as porous microspheres in hydrothermal process. Zn-citrate complex chemisorbed on the nanosheet improved the thermal stability of Zn(OH)2, which means a decomposition temperature over 200 ℃. Nanostructure porous ZnO microspheres were obtained by heating Zn(OH)2 microspheres at 300 ℃ and the nanosheet structure was maintained.