A LED string driving circuit includes a plurality of LED strings; a power supply unit configured to receive an input voltage and accordingly output a driving voltage to the LED strings; a comparison unit configured to receive the input voltage and output a first control signal according to a relationship between the input voltage and a predetermined voltage; a control unit configured to receive the first control signal and generate a second control signal according to a potential level of the first control signal; and a switch unit including a plurality of switch elements, and each are controlled by the second control signal and thereby forming various electrical loops and thereby configuring the LED strings corporately to form either a parallel loop or a series loop via the switch unit with respective electrical loops. A LED string driving method is also provided.