A power up detecting system for generating one of a first power up detecting signal and a second power up detecting signal as the final power up detecting signal, according to power provided by a power supplier. The power up detecting system comprises: a power up detecting module, controlled by a control signal to generate the first power up detecting signal in a first mode and to generate the second power up detecting signal in a second mode, wherein a voltage level of the first power up detecting signal is transited when the power reaches a first predetermined voltage value, and the voltage level of the second power up detecting signal is transited when the power reaches a second predetermined voltage value; where the first predetermined voltage value is higher than the second predetermined voltage value.