The invention discloses an initial phase angle determining method and system. The determining method and system are applied to measurement systems used for performing space charge measurement on insulating materials under alternating current voltages. The determining method includes the following steps that: a power-frequency alternating voltage of a preset voltage level is applied to an insulating material to be measured; a pulse signal of a preset frequency is applied to the insulating material to be measured; the acoustic signals of the insulating material under the influence of the power-frequency alternating voltage and the pulse signal are detected; the acoustic signals are sampled, a plurality of charge density signals corresponding to preset detection time points in a complete power-frequency period are obtained; the plurality of charge density signals are fitted according to a preset fitting equation, so that a sine curve can be obtained; and the phase angles of the space charges of the insulating material at each detection point can be directly obtained through observing the sine curve. With the method and system of the invention adopted, the initial phase angle of the space charge measurement of the insulating material under an under alternating condition can be accurately determined, and therefore, the accuracy of space charge measurement performed on the insulating material under the alternating current voltage by can be ensured.