A backlight driving circuit includes a LED driving chip (10) for outputting a PWM signal, a power port (20), a voltage conversion unit (30), a first, a second, and a third switch unit (50, 60, 70), and a level conversion unit (40). The voltage conversion unit (30) includes a primary unit (31) and a secondary winding (32). The first switch unit (50) is connected to the power port (20) and the primary unit (31). The second switch unit (60), the secondary winding (32) and a first LED string (21) forms a first circuit loop (LP1). The first and the second switch unit (50, 60) are turned on or off when receiving the PWM signal. The third switch unit (70), the primary winding (31), and a second LED string (22) form a second circuit loop (LP2). The voltage conversion unit output a reversed PWM signal to the third switch unit (70).