Hydrothermal method was invented by the geologist simulating the natural mineralization in mid-19th century and later turned to study functional materials. Currently hydrothermal method has prepared hundreds of kinds of crystals. Hydrothermal method, also known as hydrothermal method, is the scope of liquid chemical method. It means in a sealed pressure vessel, carriing out at high temperature and pressure conditions of the chemical reaction. Hydrothermal reaction depends on the type of reaction and it can be divided into hydrothermal oxidation, water and heat reduction, hydrothermal precipitation, hydrothermal synthesis, water hot solution, hydrothermal crystallization.

The Main Steps:
1. Dissolving the 3.5gNa₂WO₄•2H₂0 in 20ml of distilled water, dubbed the solution;
2. Exchanging resin column to produce hydrated tungstate sol by the solution that was hydrated;
3. Stirring constantly the dropped sol with a volume of 70ml glass barrel;
4. Adding to the resulting sol, and mixing well under 4.84g of citric acid.

Conclusions:
Preparing quadrature phase WO₃ nano-sized rectangular sheets by hydrothermal synthesis with citric acid as a controlling agent, which shows there is a great relationship between the structure of product and the reaction temperature. X-ray diffraction results show that the reaction product is a WO₃ • H₂O and a small amount of monoclinic WO₃ (γ- WO₃); with the reaction temperature, the product of the following occurs transition: WO₃ • H₂O → WO₃. SEM results showed that the product obtained are rectangular sheet structure at the reaction temperature. They are less than the surface area, between 8 ~ 9 m2 • g-1. We can control the product's composition, morphology and specific surface area by adjusting the citric acid and the amount of substance ratio (nCit / nTA). When nCitn / TA = 0, the product of h-WO3 and WO₃ • 1 / 3H₂O is composition; when nCit / nTA = 0.5 and nCit / nTA = 1, the product is γ-WO₃ morphology rectangular sheet. In the case of citric acid was added, than the surface area of the product decreases with an increase of nCit / nTA.