An apparatus and a method for driving the light source of a projector are provided. The apparatus includes an adjustment unit, a lamp driver, a phase lock loop (PLL) and a mercury lamp. The adjustment unit provides an enabling signal and regulates the frequency of the enabling signal according to an indication signal. The lamp driver provides a driving signal according to the enabling signal and an input voltage, wherein the frequency of the driving signal is varied with the frequency of the enabling signal. The PLL provides a synchronized driving signal according to the driving signal and a vertical synchronization signal, wherein the synchronized driving signal is phase-synchronous with the frame field of the projector. The mercury lamp serves as the light source of the projector according to the synchronized driving signal.