Naphtha cracking bottoms(NCB) oil was reformed by varying the heat treatment temperature, treatment time, and nitrogen flow rate in preparation of precursor pitch for isotropic pitch-based carbon fibers and activated carbon fibers. The reformed pitches were investigated in the yield, softening point, elementary analysis, and molecular weight distribution, and then the precursors reformed were melt spun to certify the optimum reforming conditions. The optimum precursor pitch was prepared when the NCB oil was reformed at 380 ℃, 3 h and 1.25 vvm N2, and it’s the softening point was around 240 ℃. The reforming resulted in product yield of 21 wt％. The C/H mole ratio of the precursor pitch increased from 1.07 to 1.34, the aromaticity increased from 0.85 to 0.88. The insolubles in benzene and quinoline were 30.0 wt％ and 1.5 wt％, respectively. The spinning temperature was about 50 ℃ higher than the softening point. The molecular weights of the precursor components were distributed from 250 to 1250, and 80％ of them were in the range of 250 to 700.