Many rivers of Western Taiwan suffer from severe bedrock erosion, especially in the geological province of the Western Foothills with formations composed of young sedimentary rocks. The problem of weak bedrock erosion has caused casualties in several bridge failures and threatened the stability of hydraulic structures. These types of structures damages mostly occurred during large flood with a high sediment concentration. Toward understanding the effect of sediment concentration on the erosion of the weak rock, a series of submerged jet tests is conducted in this study. The objective of the original submerged jet test is to determine the threshold of shear stress parameters for the characterization of soil erodibility. We modified the jet apparatus to increase the jet pressure for weak rocks. Also, a point gauge was installed with a vernier caliper for measuring the small change of the scour depth. Three types of turbidity water with the sediment concentration of 1000 ppm, 5000 ppm, 10000 ppm were prepared for jet tests. A series of laboratory jet tests was performed on the specimens of weak sandstone with the three kinds of turbidity water and clean water. Also, one set of field test was conducted at one river bank, which is composed of weak shale, using river water (< 1000 ppm) and 10000 ppm turbidity water. The laboratory results show that the average erosion depth of jetted hole for the water being clean, 1000 ppm, 5000 ppm and 10000 ppm are 0.16 mm, 0.25 mm, 0.35 mm, and 0.46 mm, respectively. It reflects the fact that the depth increases with the increase of sediment concentration. The field test results show that the jetted-hole depth for river water and 10000 ppm water are 0.44mm and 2.94 mm, respectively. Both results can be inferred that the influence of sediment concentrations on the erosion of weak rock is strong.