The invention provides a one-
step method used for synthesis of an iron-based 
molecular sieve catalyst. The one-
step method comprises following steps: a Fe-complex solution and an 
aluminosilicate molecular sieve gel 
system are prepared respectively; the Fe-complex solution is added into the 
aluminosilicate molecular sieve gel 
system slowly, full stirring is carried out, an obtained mixture is allowed to stand for aging, is introduced into a reaction kettle for 
crystallization, is washed with deionized water, and is subjected to pumping 
filtration, 
drying, and 
roasting so as to obtain a 
solid iron-based molecular 
sieve; and then 
ammonium exchange is carried out so as to obtain the iron-based molecular 
sieve catalyst. The technology of the one-
step method is simple; adoption of a plurality oftimes of 
ammonium nitrate and iron 
salt solution ion exchange and calcining is avoided, a defect in the prior art that later stage 
ion exchange technology is needed for loading of the 
active component is avoided, the synthesized iron-based molecular 
sieve catalyst is better in catalytic activity, Fe distribution is more uniform, excellent NH3-SCR catalytic activity is maintained in a wider temperature window, and at the same time, excellent high temperature activity and N2 selectivity are achieved.