Abstract: The reduction of A6 ammonia synthesis catalyst under atmospheric pressure and at temperatures 435-530℃ has been studied. It is found that when the reducing temperature is below 515℃, the catalyst cant be reduced completely but only to a limited extent. At 435℃, 490℃and 515℃, the maximum extent of reduction corresponds to 22%, 82%, and 100% respectively. The result of X-ray structure analysis shows that before reduction the catalyst has a structure of spinel and changes to (a)-Fe after reduction. The portion of the spinel which appears more difficult to be reduced is smaller in its lattice constant. A series of activity tests under 300 atms for samples with different extent of reduction have been carried out, and the constant of reaction rate with Tern-kin Equation has been calculated. It is found that the portion of the catalyst which appears more difficult to be reduced possesses higher specific activity after reduction.