Abstract: Based on the (Ⅰ) of the present work, the behavior of shear beam model at crack initiation stage and at instable propagation stage was studied. The prime results include: 1) discriminant equation which clarifies the mode of instability, snap-back or snap-through, was established; 2) analytical solution was given out for the double shear beam and the load-displacement diagram for monotonic loading was presented for a full process; and 3) the problem of the energy release induced by instability was discussed.