The present invention relates to a composite catalyst, preparation process thereof, and process for catalyzing the trimerization of butadiene using the composite catalyst. The composite catalyst comprises: (A) a titanium compound catalyst active component, (B) an organometallic compound co-catalyst component, (C) a sulfoxide compound catalyst-modifying component, (D) a monoester compound catalyst-modifying component, and (E) a solvent component. The composite catalyst has advantages of excellent selectivity, high catalytic activity, easy preparation and so on.