A slowdown subduction model is presented to estimate slab breakoff depth and initial time when continental lithosphere subducts into mantle. By assuming oceanic lithosphere subducts with uniform velocity (all forces acting on plate are balanced) and noticing that, to leading order, some resistant forces are proportional functions of subduction velocity, a simple dynamic model is proposed to describe the retarded subduction process. Thermal and strength structures within the slab are calculated under this slowdown process. We then compare the integrated strength of transitional zone from continental to oceanic lithosphere within the slab with buoyancy change due to subducted continental crust to assess slab breakoff depth and initial time. Normally, the slab breakoff depth is 65 ~ 130 km and initial time is 1 ~ 6 M.yr. after continental lithosphere initiates subduction. The initial subduction velocity is a major factor that affects the breakoff depth and initial time while subduction angle has insignificant influence.