The invention relates to an online measure and control method for processed thickness during the mirror milling process. The method comprises the following steps of: obtaining thickness data, obtaining the contour thickness information of a metal piece through an electromagnetic ultrasonic thickness measuring probe and transmitting the information to an embedded control system for online measurement and control; obtaining spindle cutter data, collecting the spindle cutter data of a device and transmitting to the embedded control system; obtaining support axis data, collecting the data of a support axis and transmitting to the embedded control system; analyzing and processing the data, fusing the obtained data through a data association network, and analyzing and processing through the embedded control system; and getting result data feedback, comparing the result data analyzed and processed by online measure and control embedded control system with pre-set processing data model, sending result data feedback to the corresponding control axis, adjusting the spindle cutter feed of a mirror milling device and the movement of the support axis to adapt the spindle cutter feed and the support shaft movement to the contour thickness of the metal part.