A compact planar circularly polarized (CP) monopulse array antenna at the S-band based on an air-filled substrate integrated coaxial line (AF-SICL) feeding structure to suppress sidelobes in both sum and difference beam is proposed in this article. The AF-SICL and the stacked patch are tightly integrated to achieve subarray sequential rotation structure with unequally spaced elements, combined with an unequal amplitude feed network with a large power division ratio contributing to reducing transmission loss and achieving a compact structure. The AF-SICL comparator network can achieve stable amplitude and phase correspondence for dual-CP antenna array. Moreover, a physical prototype of an 8×8 monopulse array antenna has been designed and fabricated. The measured results indicate that the sum and difference ports of the antenna array achieve excellent impedance bandwidth within the 8.7% (2.2-2.4 GHz), axial ratio lower than 0.3 dB, a peak gain of 23.45 dBi and the peak total efficiency is 70.2%. The symmetrical radiation pattern with a null depth less than -32.2 dB and the sidelobe levels of the sum and difference beams are suppresd to -16.2 dB and -16.1 dB respectively. The proposed monopulse array antenna holds great potential for high-precision, long-range target localization and tracking.