Micro milling can fabricate miniaturized components using micro end mill at high rotational speeds. The analysis of cutting force and stability in micro milling plays an important role in characterizing the cutting process. A numerical analysis method is presented to investigate the helical ball micro milling process with variable radial immersion. The schematic model of ball micro milling is constructed and the cutting force calculation formula is derived. And then, a detailed time domain simulation for micro milling forces between helical ball and square end mills is carried out, and the results are compared. In addition, the stability lobe of ball micro milling with low radial immersion is researched in detail through time domain and frequency domain methods. Finally, the Hopf and flip bifurcations are explored and the simulation results between variable stability cases are deeply compared and analyzed.