A special preventer for thermal recovery wells comprises a collar 1, a short joint 2, a body 3 and a movable flange 4, wherein the collar 1, the short joint2, the body 3 and a conical joint 7 are connected together from bottom to top via pipe threads, the lower end of the conical joint 7 is provided with external threads, and a central hole of the movable flange 4 provided with 12 bolt through holes 5 is sleeved on the body 3. The body 3 is an annular double-boss structure, both height and diameter of an upper boss are larger that those of a lower boss, both the upper portion and the lower portion of a central hole of the body 3 are provided with internal threads, and inside of the external circumference of the bottom face boss is provided with a steel ring groove. The special preventer bridges the gap that no special preventer is supplied for thermal recovery well construction, resolves the problems of stoppage during thermal recovery well construction, or overflowing at wellheads and difficult installation of Christmas trees in well completion, shortens well blockage time, avoids frequent standing of suspenders during stoppage, and reduces damage to suspender copper rings and fluorine rubber O-shaped seal rings. The special preventer has safe well plugging operation, saves time and labor, protects environment and has popularization value.