Cold molecules have great scientific significance in high-resolution spectroscopy,precision measurement of physical constants,cold collision,and cold chemistry.Supersonic expansion is a conventional and versatile method to produce cold molecules with high kinetic energies.We theoretically show here that fast-moving molecules from supersonic expansion can be effectively decelerated to any desired velocity with a rotating laser beam.The orbiting focus spot of the red-detuned laser serves as a two-dimensional potential well for the molecules.We analyze the dynamics of the molecules inside the decelerating potential well and investigate the dependence of their phase acceptance by the potential well on the tilting angle of the laser beam.ND3 molecules are used in the test of the scheme and their trajectories under the impact of the decelerating potential well are numerically simulated using the Monte Carlo method.For instance,with a laser beam of 20 kW in power focused into a pot of 40 μm in waist radius,ND3 molecules of 250 m/s can be brought to a standstill by the decelerating potential well within a time interval of about 0.73 ms.The total angle covered by the rotating laser beam is about 5.24° with the distance travelled by the potential well being about 9.13 cm.In fact,the molecules can be decelerated to any desired velocity depending on the parameters adopted.This scheme is simple in structure and easy to be realized in experiment.In addition,it is applicable to decelerating both molecules and atoms.