This paper was mainly about the influence of alkaline additive on the viscosity of coal water slurry (CWS). The study showed that the viscosity of CWS was sensitive to the alkaline additive. Less than 1% of alkaline additive could reduce the viscosity to more than half of the viscosity of the CWS without alkaline additive. As the content of the alkaline additive increased, the viscosity of CWS increased. The change in zeta potential, the freedom and content of the water in the CWS, the microstructure of the particles in the CWS as well as the FT-TR spectrum of the coal particles was investigated. The results show that with low concentrations of the alkaline solutions, the zeta potential of the particles in the CWS decreased which mean the absolute value of the zeta potential increased and the content of the free water in the CWS were increased. Meanwhile, the nonpolar of the particle with alkaline salts strengthened. These were all benefited to reduce the viscosity.