Small modular reactors have garnered considerable attention in the recent years. Plate fuel elementsexhibit a good application prospect in small modular pressurized water reactors for marine applications. Further, improved economic benefits can be achieved by extending the core lifetime of small modularreactors. However, it is necessary to realize a large initial residual reactivity for achieving a relatively longburnup depth finally. Thus, the selection of a suitable burnable poison (BP) is a crucial factor that shouldbe considered in the design of small modular reactors. In this study, some candidate BPs are selected torealize the effective control of reactivity. The results show that 231Pa2O3, 240Pu2O3, 167Er2O3, PACS-J, andPACS-L are ideal candidates of BP, and since the characteristics of BP can increase the final burnup depthof assembly, the economic benefits are gained. Additionally, an optimal combination scheme of BPs isestablished. Specifically, it is proved that through a reasonable combination of BPs, a low reactivityfluctuation during the lifetime can be achieved, leading to a large final burnup depth