A tristate operating mode setting device is proposed, which is designed for use with an electronic circuit unit for providing the electronic circuit unit with a tristate operating mode setting function, and which is characterized by the utilization of a specially-designed logic circuit and logic control signal generator to allow the electronic circuit unit to be selectively set to one of three different operating modes during startup through a connecting pad that can be externally connected in three different ways. This feature allows one single pad for the provision of three different operating mode settings, whereas prior art is only capable of providing two different settings. The electronic circuit unit is therefore able to use fewer number of pads to provide an increased number of operating mode settings, with the benefit of reducing layout space on circuit board.