Background: With the development of high energy physics

high performance of electron linear accelerator is required for large collider

FEL and high brightness synchrotron radiation light source.Structure asymmetry of single coupler destroys the symmetry of field distribution in coupled cavity

which reduces the quality of beam.Purpose: Optimize the asymmetry of field distribution in coupled cavity and improve the quality of beam.Methods: The simulation designs are made for single offset coupler

double symmetry coupler and the new coupler loaded by dielectric rods at X band by using CST microwave studio code.Results: The results show that the distribution of field in coupled cavity is better and all particles almost locate at the center of beam hole after beam passing through the coupler loaded by dielectric rods.The energy spread has also been significantly improved.Conclusions: The coupler loaded by dielectric rods can optimize the asymmetry of field distribution in coupled cavity and improve the quality of beam.