Fracturing behavior of rock is significantly affected by water. To study the influence of water on the critical values of stress intensity factors (the SIFc, including K_If and K_IIf) corresponding to the onset of fracture of rocks, edge-cracked semicircular bend (SCB) testing was performed on sandstone specimens (collected at a site in Linyi City, Shandong Province, China) with different water contents. First, water absorption was tested to study the evolution of the water content in the specimens. Then, the SIFc of SCB specimens after different soaking durations in water (yielding conditions ranging from dry to saturation) was tested under three-point loading. The results showed a linear decrease in SIFc with increasing water content for all the specimens with different notch angles; however, with increasing notch angle, the K_If exponentially decreased, while the K_IIf increased. To study the effect of sustained load on SIFs of rock in water, a new experiment was designed on the specimen subjected to loading while soaking for a certain duration, then the SIFc was tested. Comprehensive predictive empirical relationships are established between SIFc and water content/soaking duration for the sandstone. A novel observation is for two specimens with the same water soaking duration, the SIFc of a specimen with loading during soaking is lower than that when it was tested after soaking, which is attributed to the fact that water is more likely to penetrate into the rock, because the bending load opens pores and cracks then leads to an increase in water content under such conditions. In other words, the water content of sandstone specimens used in this study is the key factor affecting its SIFc.