The nonlinear Thomson scattering by an electron in a focused circularly polarized laser beam is investigated and a novel scheme for single attosecond pulse generation is proposed.Through computer simulation we find that by using a tightly focused laser beam,the ratio of the highest peak pulse and the second highest peak pulse of the radiation which is defined as the signal-to-noise ratio can be efficiently increased, such that a single attosencond pulse rather than a train of attosecond pulses is obtained. The pulse duration of the radiation decreases and the signal-to-noise ratio increases with increasing laser intensity.Simultaneously,the signal-to-noise ratio can be also improved as the beam-waist decreases.We find that a single attosencond pulse can be generated by using a few-cycle ultrashort laser pulse as well.