The invention discloses a prediction method of undeformed cutting thickness in ultrasonic milling of a non-uniform helical angle milling cutter. The method comprises the following specific steps: (1) comparing the traditional three-axle linkage milling process with the three-axle linkage ultrasonic milling process so as to derive a cutting edge trajectory direction of the ultrasonic milling; (2) modelling instant undeformed cutting thickness; (3) obtaining a point position by use of a position equation of any point on the current cutting edge trajectory, and computing the position of the former cutting edge trajectory point by use of a golden section iteration method, thereby obtaining the obtained instant undeformed milling thickness by use of the line segment minimum value between two points. Under the ultrasonic milling condition, the ultrasonic milling force is explored and analyzed by predicting the instant undeformed milling thickness; and the prediction method disclosed by the invention can be suitable for the milling procedure under a cutter with non-uniform helical angle and a complex path; after predicting the instant undeformed cutting thickness, the influence on the cutter optimization and work-piece machining by the ultrasonic milling force can be further understood.