A multi-chip stack structure and a fabrication method thereof are proposed, including providing a leadframe having a die base and a plurality of leads and disposing a first and a second chips on the two surfaces of the die base respectively; disposing the leadframe on a heating block having a cavity in a wire bonding process with the second chip received in the cavity of the heating block; performing a first wire bonding process to electrically connect the first chip to the leads through a plurality of first bonding wires, and forming a bump on one side of the leads connected with the first bonding wires; disposing the leadframe in an upside down manner to the heating block via the bump with the first chip and the first bonding wires received in the cavity of the heating block; and performing a second wire bonding process to electrically connect the second chip to the leads through a plurality of second bonding wires. The bump is used for supporting the leads to a certain height so as to keep the bonding wires from contacting the heating block and eliminate the need of using a second heating block in the second wire bonding process of the prior art, thereby saving time and costs in a fabrication process. Also, as positions where the first and second bonding wires are bonded to the leads on opposite sides of the leadframe correspond with each other, the conventional problems of adversely affected electrical performance and electrical mismatch can be prevented.