Columnar jointed basalt develops in the arch dam foundation and the dam abutment of Baihetan Hydropower Station. After the water storage in the hydropower station, the head difference between the upstream and the downstream of dam will form a high seepage pressure, and the strength characteristics of rock mass under high osmotic pressure is one of concerned scientific issues in rock engineering. Therefore, artificial material with physical and mechanical characteristics similar to those of in-situ rock mass was used to make the columnar jointed rock mass model samples with the same joint structure. In order to investigate the effect of seepage flow pressure on the failure modes, strength and seepage quantity of jointed rock mass, a series of triaxial tests on the columnar jointed rock mass samples were carried out under different osmotic pressures. The experimental results indicate that the failure mode of columnar jointed rock mass is sliding failure which alongs the joint dip angle under seepage flow pressure. Moreover, the columnar jointed rock mass has a lower strength under seepage, and the peak strength of triaxial compression test is negatively correlated with the osmotic pressure. Furthermore, four stages for the stress-strain curve of columnar jointed rock mass under triaxial compression condition are summarized based on test results, and seepage quantity shows phased characteristics with the stress-strain curve. Additionally, the effect of seepage on columnar jointed rock mass strength and seepage flow is obvious.