Although the general carbon fibers are very stable, low-temperature carbonized carbonaceous fibers were sensitive to moisture, which would lead to unstable electrical conductivity, and seriously endanger the antistatic properties of antistatic coating when carbonaceous fibers were used as conductive filler. In this work, a simple dipping modification is proposed to improve the stability of fiber resistivity by using a long carbon chain silane coupling agent (hexadecyltrimethoxysilane) to perform surface moisture-proof treatment on carbonaceous fibers. The modification was demonstrated to significantly improve the stability of fiber resistivity by accelerated experiments in hygrothermal and salt spray environments. After 72 h treatment in hygrothermal environment, the increase percentage of untreated carbonaceous fiber resistivity reached 70%. Nevertheless, that of CF@KH1631-6 was 40%. Good hydrophobicity was achieved at a concentration of 6 wt%, water contact angle increasing from 135.6° to 152.2°. Furthermore, the whole preparation process had the potential of large-scale industrial production and application.