Conventional reflectarrays adopt phase shifters and microstrip lines to form the beam pattern. To reduce the cost of phase control and system complexity, a beamforming technology based on time-modulated reflectarrays is introduced. The time modulated reflectarray is a kind of reflectarray antennas with simple reconfigurable reflecting elements, which can be utilized to realize many functions including sidelobe suppression and beam forming. Compared with traditional reflectarray which use microstrip lines and phase-shifting devices, the time modulated reflectarray can control the output of array by controlling the switch-on and switch-off times of reflection elements. The theoretical analysis and simulation results show that the time-domain modulated reflectarray can generate the ideal directional pattern and realize the beam forming function without adopting the phased-array units and the cooperative feeding network in the traditional phased reflectarrays. This feature will facilitate the application of beam forming technology in the millimeter-wave reflectarrays.