The water uptake and main controlling factors on the organic-rich mudstone of Longmaxi Formation are investigated by water absorption experiment, N2 absorption and TOC and clay mineral contents. The results show that TOC and clay mineral contents are key factors that affect water absorption on mudstone. Water uptake in mudstone has a positive correlation with BET surface area, and the water uptaking in mudstone absorption is spontaneous and reversible. Based on the linear correlation between saturated water uptake of the same sample with different sample size and volume, the calculated effective porosity is 6.23%. Water absorption experiment provides a new method to measure the connected and effective porosity. ©, 2015, Science Press. All right reserved.