The spatial distribution of Thomson scattering in an ultrashort laser pulse is investigated. It is found that the spatial distribution of the radiation depends sensitively on the carrier-envelope (CE) phase η0 of the ultrashort driving laser pulse. At η0=0,π, the spatial distribution of the radiation shows fourfold or twofold symmetry, while the symmetry breaks down at other CE phases. The collimation of the radiation also depends sensitively on the CE phase, and it reaches the best at η0＝±π/2. These results suggest that it is possible to control the spatial distribution of Thomson scattering by varying the CE phase of the driving laser pulse, or the CE phase of the ultrashort laser pulse can be measured by utilizing the phase-dependent spatial symmetry of Thomson scattering.