The invention discloses a magnetic cobalt (II) complex based on a 4,4'-bipyridine-itaconic acid derivative ligand and a preparation method of the magnetic cobalt (II) complex. The molecular formula ofthe magnetic cobalt (II) complex is [[Co(HL)2(H2O)2].2NO3]n, wherein HL represents the 4,4'-bipyridine-itaconic acid derivative ligand with one hydrogen atom removed from carboxylic acid, a single-crystal structure has a one-dimensional chain structure, and an asymmetric construction unit of the single-crystal structure contains half cobalt (II) ion, one 4,4'-bipyridine-itaconic acid derivative ligand, one end coordination water molecule and one free nitrate anion; pyridine nitrogen in 4,4'-bipyridineis subjected to an electrophilic addition reaction with double bonds in itaconic acid, a semi-rigid carboxylate ligand containing N(+)-C bonds is synthesized, and the ligand and the cobalt (II) ion are subjected to an in-situ coordination reaction, and the cobalt (II) complex is synthesized;the cobalt (II) complex has slow magnetic relaxation phenomenon at the low temperature and serves as a molecule-based magnetic material to be applied to the fields of information storage, quantum computation and the like.