The present invention relates to an application of a molecular size selective catalyst in a Knoevenagel condensation reaction, and belongs to the field of fine chemical engineering synthesis. The present invention provides a heterogeneous catalyst, ie., a microporous metal organic framework material Cu-HKUST-1 immobilized alkaline ionic liquid, and an application of the heterogeneous catalyst in condensation reactions of aromatic aldehydes with different sizes and malononitrile. At a room temperature, the novel heterogeneous catalyst provides selective catalysis performances for substrates with different molecule sizes, wherein the characteristic is suitable for preparation of high selectivity target product in the continuous reaction for fine chemical synthesis.