With the advances of manufacturing technology and aggressive use of multi-level-cell (MLC) for flash memory, flash memory has faced a serious challenge on its device lifetime due to the fast-decreasing capacity caused by worn-out blocks. In contrast to existing works, we propose a block reinforcement scheme to optimize the guaranteed lifetime of flash storage devices by extending the useable period of flash blocks with limited performance degradation and space overhead. A serious of experiments was conducted to evaluate the efficacy of the proposed scheme and the results are very encouraging.