We propose an efficient terahertz radiation scheme using electron-beams to excite cascaded gratings, which can transform a series of surface waves on the cascaded gratings into coherent Smith-Purcell Radiation (SPR). Compared with the conventional SPR schemes, its radiation efficiency and power can be orders of magnitude higher. In addition, it changes the conventional incoherent SPR into coherent emission with frequency being continuously tunable. Thus, it affords a promising way of developing high power, coherent, and tunable terahertz radiation sources.