An apparatus for prolonging lamp lifetime by reducing luminance of a least one lamp (5) in phases includes an image signal outputting unit (1), a scaler (2), and a Microprogrammed Control Unit (MCU) (4). The MCU further includes a CPU (41), a timer (42), and a memory (43). The image signal outputting unit outputs image signals to the scaler. The MCU detects in real-time statuses of the image signals being input to the scaler. The timer records each elapsed time "Tn" during which no image signal is input to the scaler. The CPU compares the elapsed time "Tn" with preset times stored in the memory, and sends a control signal to the scaler according to the comparison result. The scaler generates PWM waves with a corresponding duty cycle according to the control signal, in order to control the luminance of the lamp. A related method for prolonging lamp lifetime is also provided.