A voltage adjusting circuit is provided. The voltage adjusting circuit for adjusting the output voltages supplied by voltage sources includes a test control device, a multiplexer, a comparator, and a built in self test (BIST) device. The test control device selects one of the voltage sources as a testing voltage source, and outputs a selecting command for selecting the testing voltage source and a target voltage corresponding to the testing voltage source. The multiplexer is coupled to the voltage sources, receives an enablement signal, and outputs a voltage supplied by the testing voltage source as a testing voltage according to the enablement signal. The comparator compares the voltage levels of the testing voltage and the target voltage, and outputs a comparison result. The BIST device receives the selecting command, outputs the enablement signal for enabling the testing voltage source according to the selecting command, and adjusts the voltage supplied by the testing voltage source to a predetermined voltage according to the comparison result.