An AC power source controller includes an input side, which is electrically connected to an output side via a power source controlling interface; and a phase modulator, which is electrically connected to the power source controlling interface. The phase modulator includes a microcontroller unit, an encoding DIP switch, a control signal receiving module, and a signal detecting interface, which may accurately detect the phase variation of an input power source so that, according to chosen conducting phase Pn, the microcontroller unit may automatically adjust the time delay Dn between conducting and cutting off orders sent to the power source control interface to control the power source output accurately, whereby the inaccuracy and instability of AC power source output control and phase difference between current and voltage generated by reactance load may be improved.