Producing lower olefins via iron-based Fischer-Tropsch synthesis without intermediate process has received increasing interests. Herein, effects of support modification on FTO performance of CNT supported Fe catalysts are comparatively investigated. Employing the defect-rich CNT as the catalyst support compared to the pristine CNT support is found to exhibit the shorter induction period and higher iron time yield (FTY). The promotional effects of K and Mn promoters on the FTO performance are subsequently revealed on the defect-rich CNT supported Fe catalysts. Both promoters are observed to suppress the formation of undesirable methane and to enhance the selectivity to desirable lower olefins. Moreover, the introduction of the Mn promoter facilitates the dispersion of iron particles and thus shows higher FTY, and it also gives rise to much higher catalyst stability than the K promoter. The insights reported here could guide the design and optimization of promoted Fe/C FTO catalysts.