An inorganic/organic composite support includes an inert inorganic material, a complex compound composed of a polylol and a magnesium halide, and an organic polymer containing a polar functional group. Said complex compound loaded with the organic polymer is deposited on the inert inorganic material. When the inorganic/organic composite support used as a polyolefin catalyst carrier is applied to ethene or propylene polymerization, the fusion-flow ratio of the obtained polymerisate is remarkably increased and its processing performance is improved. When the catalyst is applied to ethene polymerization, the fusion-flow ratio of the polymerisate can be adjusted by changing the content of hydrogen. When the catalyst is applied to ethene or propylene polymerization, the catalyst activity is high.