The author describes the basic situation of unstability of drift in poor strata,analyzes the unstability causes of this kind of drifts, and points out,on the hand,the function, which the soft rockbody was forced by geological stress,and on the other hand the disturbance, which the soft rockbody was suffered from the blasting force in excavating, make the stress field change dramatically,the host rock dismantle arid move, and further the drift so unstable that it will collapse and fall. At last,the author makes an approch to the measures used for controlling the unstability of drift.It is considered that the control of host rock dismantlement and movement should be started from enhancing the self-support capability of host rock. Meanwhile, a new support structure is used to make the host rock, on the whole, have a higher supporting capability for the surrounding pressure.