An exemplary method for adjusting clock phase of a monitor includes: setting a reference threshold voltage, when an input voltage of image signals is greater than the reference threshold voltage, a scaler begins to receive clock phases generated by a Phase Locked Loop; dividing the clock phase into a plurality of equal periods, recording a corresponding input voltage at each point dividing two adjacent period, and setting the corresponding input voltage as a threshold voltage of the next period; recording a quantity of the clock pulses in each period; evaluating whether a period of the clock phase is a regular period according to whether the quantity of clock pulse in the period is equal to a reference quantity or not, while the input voltage is generating retardation; and selecting the input voltage of the image signals corresponding to a regular period as a threshold voltage of the scaler.