An absolute time-grating angular displacement sensor based on an alternating electric field, comprising a stator substrate and a rotor substrate mounted coaxially with the stator substrate, wherein a lower surface of the rotor substrate and an upper surface of the stator substrate are opposite to and parallel with each other and have a gap therebetween; the lower surface of the rotor substrate is provided with an induction electrode, and the upper surface of the stator substrate is provided with an excitation electrode opposite to the induction electrode; the excitation electrode is composed of a circle of sector-annular electrodes having the same radial height and the same central angle, and arranged at equal intervals in a circumferential direction, wherein the (4 n 1 +1) th sector-annular electrodes are connected into a group to form an excitation phase A, the (4 n 1 +2) th sector-annular electrodes are connected into a group to form an excitation phase B, the (4 n 1 +3) th sector-annular electrodes are connected into a group to form an excitation phase C, and the (4 n 1 +4) th sector-annular electrodes are connected into a group to form an excitation phase D, wherein n 1 is sequentially valued from all integers from 0 to M 1 -1, and M 1 represents the number of periods of the excitation electrode, wherein the upper surface of the stator substrate is provided with a differential reception electrode I and a differential reception electrode II, the reception electrode I is located outside the excitation electrode, the reception electrode II is located inside the excitation electrode, and the lower surface of the rotor substrate is provided with a reflection electrode I opposite to the reception electrode I and a reflection electrode II opposite to the reception electrode II; the induction electrode is composed of a circle of identical double-cosine-shaped electrodes arranged at equal intervals in the circumferential direction, and the radial height of the double-cosine-shaped electrode is smaller than that of the sector-annular electrode, wherein the (4 n 2 +1) th double-cosine-shaped electrodes are connected into a group to form an induction group A, the (4 n 2 +2) th double-cosine-shaped electrodes are connected into a group to form an induction group B, the (4 n 2 +3) th double-cosine-shaped electrodes are connected into a group to form an induction group C, and the (4 n 2 +4)th double-cosine-shaped electrodes are connected into a group to form an induction group D, wherein n 2 is sequentially valued from all integers from 0 to M 2 -1, and M 2 represents the number of periods of the induction electrode; the reflection electrode I has a reflection group A 1, a reflection group B 1, a reflection group C 1, and a reflection group D 1 which are respectively connected to the corresponding induction groups A, B, C, and D; the reflection electrode II has a reflection group A 2, a reflection group B 2, a reflection group C 2 and a reflection group D 2 which are respectively connected to the corresponding induction groups A, B, C, and D; during measurement, the rotor substrate and the stator substrate rotate relatively in parallel, and four channels of co-frequency constant-amplitude sinusoidal excitation voltages with a phase difference of 90° are applied to the excitation phases A, B, C, and D of the stator substrate respectively; first and second co-frequency constant-amplitude traveling wave signals with a phase difference of 180° are generated on the reception electrode I, and third and fourth co-frequency constant-amplitude traveling wave signals with a phase difference of 180° are generated on the reception electrode II; an precisely-measured first-channel sinusoidal traveling wave signal is generated by combining the first traveling wave signal with the second traveling wave signal through a subtraction circuit, and an precisely-measured second-channel sinusoidal traveling wave signal is generated by combining the third traveling wave signal with the fourth traveling wave signal through a subtraction circuit; the precisely-measured first-channel sinusoidal traveling wave signal or the precisely-measured second-channel sinusoidal traveling wave signal is processed to obtain an precisely-measured angular displacement value; a phase difference generated after the phase comparison between the precisely-measured first-channel sinusoidal traveling wave signal and the precisely-measured second-channel sinusoidal traveling wave signal is processed to obtain a roughly-measured electrode position count value; and the precisely-measured angular displacement value is combined with the roughly-measured electrode position count value to obtain an absolute angular displacement value.