Objectives. To explore which and how many root canal(s) should be inserted fiber-reinforced composite (FRC) post(s) in for an endodontically-treated maxillary first molar with a disto-occluso-palatal (DOP) defect. Methods. Eight different models of the maxillary first molar were established. In the multi-post groups, the thinner post was horizontally trimmed 1 mm below the intersection point as the assistant post, and the thicker one was named the main post. Two types of forces were applied: one is a vertical force of 800 N parallel to the long axis of the tooth and the another one is a lateral force of 225 N directed at 45° to the long axis of the tooth. The equivalent stresses on the external surfaces of the tooth tissue, the internal surfaces of the root canals and in the posts, as well as the maximal shear stresses on the post-cement interface and the cement-dentin interface were calculated with finite element analysis (FEA). Results. All the models showed similar distribution patterns of equivalent stress on the external surfaces of the tooth tissue under the same loading. The maximal equivalent stress values were found on the palatal (P) side of the root trunk under vertical loading, and on the mesiobuccal (MB) side of the root trunk under lateral loading. On the internal surfaces of root canals, the maximal equivalent stress value increased at the middle third of the post-placed canal, and decreased at the cervical third of the canal. Under vertical loading, P posts showed the highest equivalent stress values, while MB posts showed the highest equivalent stress values under lateral loading. Under vertical loading, the main post encountered larger shear stress than the assistant one on the post-cement interface in the same canal. Under lateral loading, the post-cement interface in the MB canal showed the highest shear stress values. The highest shear stress values appeared on the cement-canal interface with a P post under vertical loading, and with a MB post under lateral loading. Conclusion. The FEA results indicated that the restoration with FRC post could conduct the occlusal force towards apical area, so as to improve the stress distribution in the residual tooth tissue for a maxillary first molar with DOP defect. One post in the P canal may be the best strategy of FRC restoration for such a tooth.