Abstract: Provided are a process for preparing an N ortho acyl substituted nitrogen-containing heterocyclic compound and an aminal iron (II) complex thereof, and the use of the complexes obtained by the process in an olefin oligomerization catalyst. The N ortho acyl substituted nitrogen-containing heterocyclic compound in the present invention is for example 2-acyl-1,10-phenanthroline or 2,6-diacetyl pyridine as shown in formula b, and the N ortho acyl substituted nitrogen-containing heterocyclic compound in the present invention is produced by a reaction of a precursor thereof in a substituted or unsubstituted nitrobenzene. Preferably the precursor shown in formula I in the present invention is produced by 1,10-phenanthroline reacting with trialkyl aluminum, or a halogenoalkyl aluminum RnAlXm, or a substituted or unsubstituted benzyl lithium 2Li, followed by hydrolysis.