The method of UV/Visible absorption spectroscopy for olefin catalytic system was introduced in this paper, whose testing condition was much closer to the polymerization conditions. The actions of olefin polymerization catalyst (dbm)2 ZrCl2 with cocatalyst AlEt2 Cl (or MAO) were investigated by UV/visible absorption spectroscopy at atmosphere temperature. It was shown that the UV/Visible main absorption band of the zirconocenium, which can be related to the ligand to metal charge transfer bands (LMCT), varies greatly upon incremental addition of AlEt2 Cl or MAO. For the low molar ratios of Al/Zr in the catalytic system, there was the substitution of an electron withdrawing chlorine atom by a donating alkyl group. Then a hypsochromic shift of the initial catalyst absorption band, corresponding to the monomethylation of the catalyst, was observed in each catalytic system (dbm)2 ZrCl2/AlEt2 Cl (or (dbm)2 ZrCl2/MAO). On the contrary, further addition of AlEt2 Cl (or MAO) was accompanied by a continuous bathochromic shift of the maximal wavelength, which corresponding to the formation of more dissociated ionic active species. Then, there would be a coordination of monomer to the ionic active species.