In order to improve the engineering properties of organic soil, a new stabilization agent is developed by the addition of phosphor gypsum and calcium aluminate cement. The artificial organic soil is applied in the study and a series of laboratory tests were carried out to explore new stabilization agents and determine the optimal dosage. Unconfined compressive strength (UCS) and the pH value of soil pore solution were measured. The influence of organic content, agent composition and curing time on the UCS of samples were also researched. The test results show that the UCS of stabilized organic soils by a new agent achieves approximately 800 and 1 200 kPa at 28 and 90 d curing time, respectively. The pH test results show that a high alkaline environment is a necessary and not a sufficient condition for high strength. The strength of stabilized soil is related to the hydration product of stabilization agent. The mechanism of strength formation was also explored by X-ray diffraction (XRD), mercury intrusion porosimetry(MIP) and scanning electron microscope (SEM) tests. A large amount of ettringite is produced to fill the large pores of organic soils, which contribute to the high UCS value of stabilized organic soils. The new agent can solidify the organic soil successfully as well as provide a new approach to treat the organic soil.