This paper presents a design of a gyro-TWT operating in the large-orbit electron beam mode, with the aim of reducing the working magnetic field while ensuring operational efficiency. The periodic dielectric-loaded structure is adopted as the high-frequency interaction circuit, which has not been reported in the application of large-orbit gyro-TWT in literature, while this structure has been successfully applied in the development of small-orbit devices. This paper conducts a comprehensive study and analysis of this structure and achieves stable operation in the Ka-band after the optimization of the tube. This tube works at second harmonic of electron frequency in the mode of large-orbit electron beam. The required magnetic field is only 5100 Gauss, which can be generated using electromagnetic coils instead of superconducting magnets. The operational parameters include voltage of 75 kV, current of 9A, and velocity spread of 3.5%. Under these conditions, the device presents stable operation, with -3 dB bandwidth of 4.3 GHz, and maximum output power of 165 kW. This result meets the expected requirements for magnetic field and operational efficiency, thus validating the feasibility of practical fabrication of large-orbit gyro-TWT with periodic dielectric-loaded structure.