Flexible parts are widely used in the aeronautic and power industries which have high machining accuracy requirement. However, the flexible part is apt to vibrate in cutting processes due to its low structure stiffness and damping. Accurate approximation of cutting stability boundary of flexible part is difficult, because traditional stability analysis simplifies the complex dynamic system and some parameters still have large vibrations and can cause bad surface accuracy even when they are predicted to be located in the stable zone. To better evaluate the stability in flexible part milling, a synthetical stability method is proposed to calculate the boundary which can assure both the cutting stability and the surface location accuracy. To exclude other factors which may have great effect on cutting stability, such as multiple structure modes and structure mode coupling, etc, a flexible support platform is designed to test the validity of the classical local stability analysis. After the comparison between the local stability prediction and the experimental results, it is found that the local stability analysis is defective for the stability prediction of the flexible part milling. Therefore, the dynamical equation is improved and both the periodical excitation and the regenerative excitation are considered. The stability boundary is calculated by using the time-domain numerical method, with the radial immersion and cutting entrance angle's variation originated from forced vibration being considered. The simulation based on the synthetical stability analysis has a good agreement with the experimental results, which verifies the validity of the presented method.