High-performance aramid fiber sheets are a new class of composite materials made up of weaved polyamide fibers. In this study, the seismic performance and failure behavior of timber column-ground sill joints reinforced with aramid fiber sheets were investigated. In a past study, we conducted bending tests under cyclic loading for three column-ground sill specimens. After reinforcing the specimens with aramid fiber sheets, the joint strength improved but was dependent on the method of attaching the sheet. It was found that the compression zone of the aramid fiber-reinforced plastic layer broke at the joint boundary. In this paper, we proposed an improvement in the method of attaching the fiber sheet to the joint. On the compression zone at the boundary of the joint, resin was not pasted onto the aramid fiber, the fiber was not cured, and the plastic layer was not formed. Therefore, we could solve some problems and control the failure of column-ground sill joints.