This paper is proposed to reveal the stable equilibrium on the boring bar during heavy-duty deep-hole boring trepanning processing environment, including three conditions considering vortex instability caused by outside cutting fluid, perturbation instability caused by inside cutting fluid and synthesized instability accompanied both inner and outer cutting fluid, respectively. Every position of stable equilibrium and formula of rotation speed in instability are obtained. It is shown that the system instability induced by no matter how different situations, is resulted from half frequency vortex and perturbation, through case calculation and simulation on system stability.