High brightness synchrotron radiation light source and free electron laser（FEL） have higher requirements on the performance of linear accelerator coupling structure. The traditional single-side couplers induce asymmetry of electromagnetic fields in cavity due to the asymmetry of cavity structure, which greatly affects the beam quality. A new type of single-side coupler loaded with dielectric cylindrical rods is proposed. Its structure is simple, and easy to be processed. The simulation results show that dipolar and quadrupolar electric field amplitude gradient for the new coupler are －0.0794% and －0.066%, respectively, but the ones are 5.02%, 2.99% for the traditional couplers. The asymmetry of field amplitude is effectively weakened. The temperature field distribution, thermal deformation and thermal stress of coupler are analyzed and calculated. Results show that the new coupler has little difference with the usual single-side coupler in thermal performance, and is expected to improve the performance of light source.