In order to study the stability of the two dimension ultrasonic vibration milling system in vertical plane, the stability model of a two-dimensional ultrasonic vibration milling in vertical plane is established. The stability of the two dimensional ultrasonic vibration milling is studied using the full discrete method. The stability of the system is obtained by numerical simulation using Matlab software. The results of chatter test for milling titanium alloy material show that the numerical simulation results are in good agreement with the experimental results when the speed was 1 000～3 500 r/min. The correctness of the stability model of two dimensional ultrasonic vibration milling in vertical plane is verified. The stability of the system can be improved by applying two dimensional ultrasonic vibration in the vertical plane. The axial depth limit maximum of cut is increased by about 13.6%. The research fills up the blank of the two-dimensional ultrasonic vibration assisted milling stability study.