A controllable on/off device for an auxiliary light contains: a lamp module having a body in which a driving unit connecting with an illuminating element, the body including a first conducting piece coupling with a cathode contact point of the driving unit and a second conducting piece coupling with a negative contact point of the driving unit; a base fixed under the lamp module and including a conduction element corresponding to the first conducting piece and the second conducting piece; a pivotal clutch mechanism defined between the lamp module and the base so that the lamp module is axially rotated and shifted between a first rotating range and a second rotating range and drives the lamp module to move vertically, such that the first conducting piece and the second conducting piece are driven to contact with or leave from the conduction element, thus turning on or off the illuminating element.