Backward Wave Oscillator(BWO) is one of the most attractive terahertz sources belong to the vacuum electron devices, which has superior performances in power capacity, high frequency, and bandwidth. For the purpose of improving the interaction between the circular electron beam and the grating slow-wave structure in BWO, a novel slow-wave structure with double electron beams embedded in the rectangular grating is proposed. The dispersion characteristics of such structure are verified by numerical calculation and simulation part, and the results show that it can achieve higher operating frequency and coupling impedance compared with a common rectangular single grating with the same structural parameters. Moreover, PIC simulations are carried out to optimize its structure, and a stable output 10.6 W with 501 GHz frequency is obtained. This research would provide guidance for the design of 0.5 THz BWO.