Thermal decomposition method is mainstream process of preparing ammonium metatungstate (AMT) by controlling calcination temperature and ammonia and water vapor partial pressure of ammonium paratungstate (APT), so that APT is transferred to amorphous and large water-soluble substance of AMT by thermal decomposition method, and then into hydrated ammonium metatungstate solution and finally the solution was concentrated and cooled to obtain a crystalline solid ammonium metatungstate.

Ammonium paratungstate will lose part of ammonia and water under 180~260℃ to generate AMT, the chemical equation is as follow:
5(NH₄)₂O?12WO₃?H₂O=3(NH₄)₂O?12WO₃?3H₂O+4NH₃↑

The key of thermal decomposition is to control the pyrolysis temperature, because the too high temperature will form insoluble substance of tungsten oxide; while the too low temperature will cause the incomplete removal of ammonia and water of APT, and can not generate ammonium metatungstate. Advantages of pyrolysis technique is the direct use of ammonium paratungstate decomposition will bring the high purity of AMT; however, due to the extremely harsh conditions of thermal decomposition, it is difficult to accurately judge the end of reaction and often appearing burnt or under-firing problem, which extremely affects the conversion rate of thermal decomposition. Thus, researchers use an additive (strong acid cation resin) during the dissolution of pyrolysis products to completely convert the residue of ammonium paratungstate to ammonium metatungstate without affecting quality of AMT.