The invention discloses a determining method for cracking water content of swelling soil. The method comprises the following steps: (1) preparing at least two soil samples of same swelling soil and treating the soil samples till being saturated; (2) dehumidifying the saturated soil samples; (3) dehumidifying for a period of time, taking out the soil samples and measuring the water content of one soil sample, wherein the water content is an initial water content; (4) performing water penetration on the other soil sample for a period of time and determining different values of the penetrating rate of water following the change of the time; (5) repeating the steps (1) to (4), repeatedly testing and changing the dehumidifying time of the soil samples, thereby acquiring different values of the penetrating rate of the soil samples following the change of the time within the next period of time under the condition of different initial water contents; and (6) settling the test data and confirming the cracking water content of the soil samples. The determining method disclosed by the invention researches the gaps according to the water content, has repeatability and can repeatedly test according to the research requirement. The device has a simple structure, convenience in mounting and low cost.