Smith-Purcell radiation (SPR) from prebunched electron bunches moving in parallel with the surface of periodic ideal metal grating is analyzed. The influence on SPR by the energy, length, and current density distribution of electron bunches and grating structure is analyzed in detail. The radiation of multi-prebunched electron bunches is also studied. With the increase of electron bunches energy and decreases of prebunched electron bunches size and grating period, the corresponding frequency of the peak of the angular distribution of radiative energy becomes higher. For the same grating period and electron bunches parameters, triangle grating produces a higher corresponding frequency than sinusoidal grating. The less electron bunches size is, the larger the radiative energy is. The radiative energy and frequency are mainly dominated by the longitudinal length of electron bunches. Terahertz radiation will be obtained with reasonable parameters for bunches and grating.