Industrial linear accelerators are gradually moving towards high average beam power in small, compact shapes. The beam break-up effect due to the transverse wakefield is the main limitation to its performance improvement. The hybrid dielectric-iris-loaded structure is a new miniaturization accelerating structure with high average beam power. The main problem is difficulty in assembly and tuning. Through the study of dielectric based accelerating structures, a miniaturization accelerator with high average beam power was designed and optimized. During the research process, the influence of dielectric structural parameters on the accelerating structure size, accelerating gradient, and beam power was analyzed. The optimized accelerating structure size was reduced by about one-third compared to conventional iris-loaded accelerating structure. It can achieve the same acceleration gradient. The insertion of a simple dielectric tube into the dielectric structure made assembly and tuning easier. The optimized accelerator operats at S-band with the frequency of 2856 MHz and voltage of 2.5 MeV. During the research process, beam dynamics is calculated through numerical calculation methods and PARMELA. Our research provides an important groundwork for further development of irradiation linear accelerators.