General purpose pitch-based carbon fiber (PCF) was pretreated with steam to develop pores on the surface of fiber. After immersed in cobalt salt solution, PCF was used as raw materials to prepare activated carbon fiber (ACF) through steam activation process. The effect of pretreatment of carbon fiber on specific surface area, mesopore volume and pore size distribution was investigated by N2 adsorption, and morphology of the resultant ACF was observed with scanning electron microscope(SEM). The results show that pretreatment of PCF enlarges specific surface area and mesopore ratio of ACF remarkably. The best ACF obtained in experiment is the one with specific surface area of 2670 m2/g and mesoporosity of 61.8%. Cobalt has evident catalysis in preparing ACF from activation of PCF, while specific surface area and pore size of ACF get smaller with cobalt salt immersion when pretreatment is too strong.