The annular focal antenna has special electromagnetic characteristics and application fields. The phase error of annular focal antenna is analyzed theoretically and simulatedly. The phase error caused by the position deviation of the feed and the subreflector, the compensation relationship between the main reflector, and subreflector and the optical path difference caused by the operation of the antenna in holographic measurement are derived. The results will provide theoretical basis and reference for accurate surface measurement and compensation of the annular focal antenna.