The effect of inserting high-J_c HTS thin cylinders into a 200 MHz (4.7 T) nuclear magnetic resonance (NMR) superconducting bulk magnet using a field-cooled magnetization (FCM) process and three-dimensional (3D) numerical simulation was studied. High-J_c thin cylinders of various lengths and shapes were inserted in various positions. A recent experiment in which a cylinder wrapped with a high-J_c material was inserted in a bulk cylinder was confirmed to be effective in reducing the inhomogeneity of the trapped field in the hollow space. We also modeled an actual cylinder with several kinds of slits. As a result of the defect in the NMR cylinder bulk magnet, the inhomogeneity of the trapped field can be drastically improved by inserting a HTS thin cylinder that has a higher J_c than a bulk material the same length as the bulk magnet. However, the level of improvement in the trapped field homogeneity changes depending on the spatial relationship between the defect and the HTS thin cylinder, and on the length and shape of the HTS thin cylinder with and without slits. The superconducting current flows in the HTS thin cylinder to compensate for the inhomogeneous trapped field caused by the defect. The optimum length and shape of the high-J_c HTS thin cylinder is discussed in terms of realizing a practical compact NMR bulk magnet.