(a-Diimine)nickel(Ⅱ) {[C6H5 - N = C(CH3) - C(CH3) = N - C6H5]NiBr2}-TiCl4 abbreviated as NiL-TiCl4combined catalyst which is supported on MgCl2-SiO2 carrier has been prepared, by using alkyl aluminum (AlR3) as the cocatalyst in place of methylaluminoxane (MAO) to catalyze ethylene oligomerization and copolymerization in situ. The influences of procedure for supporting NiL-TiCl4, the molar ratio of NiL to TiCl4, cocatalyst type and polymerization temperature on the catalytic performance were studied. The degree of branching and the composition of the branched chain of polymers produced have been investigated by IR and 13C-NMR spectra. The results show that the combined catalyst can synthesize the branched polyethylene with various banched chains .The polymerization reaction was monitored by gas chromatography and mass spectrometry (GC-MS). The results show that this catalyst promotes the oligomerization and copolymerization in situ for ethylene.