A circuit system having compact decoupling structure, including: a mother board; at least one circuit unit, each having a substrate, a logic-circuit die, a plurality of first metal contacts, and a plurality of second metal contacts, the substrate having a first surface and a second surface, the first metal contacts being formed on the first surface and soldered onto the mother board, the second metal contacts being formed on the logic-circuit die and soldered onto the second surface to form flip-chip pillars, and the flip-chip pillars determining a height of a gap between the die and the substrate; and at least one decoupling unit for providing an AC signals decoupling function for the at least one circuit unit; wherein each of the at least one decoupling unit is placed in the gap of one said circuit unit and includes a mother die and at least one stack-type integrated-passive-device die.