A color temperature adjusting system includes a processing unit, a constant-current drive unit, and an light emitting unit (LED) unit including two unmatched LED modules with different basic color temperatures. A table records a relationship between coefficient values and current values for the current(s) flowing through the two LED modules. The processing unit selects one of a number of predetermined formulas to calculate the coefficient value by comparing a desired value with a threshold value, and further determines the current values according to the calculated coefficient value listed in a table. The constant-current drive unit includes two drive module generating modulating signals to adjust the respective values of the current flowing through the two LED modules to match the determined current values, thereby adjusting the color temperature value of the LED unit to the desired level. A related method is also provided.