The in-situ copolymerization catalyst system for preparing linear low-density polyethylene consists of oligomerizing catalyst, copolymerizing catalyst and cocatalyst. The oligomerizing catalyst is homogeneous or carried alpha-diimino pyridine iron compound, the copolymerizing catalyst is the cyclopentadienyl compound with Ti, Zr, or Hf as central atom or its carried catalyst, and the cocatalyst is alkyl aluminoxane. The catalyst system of the present invention has high catalytic activity, and the product has relatively low smelting point, 106.2-123.9 deg.c, relatively low crystallization degree, 16.7-65.8%, and density of 0.920-0.945 g/cu.cm.