Abstract: The stabilized soil strength of compound soft soil stabilizer, which is composed of cementitous component and expansible component, may be increased significantly, but also may be reduced. The causes for different stabilization effects are studied. Three compound stabilizers are used to stabilize two soil samples. The unconfined compressive strength is tested, and the generation rate of calcium silicate hydrate CSH and ettringite AFt are tested by XRD tests and DSC-TG tests. The results show that the stabilization effect of compound stabilizer depends on the coordination between AFt generation rate and CSH generation rate. If CSH generation quantity is still little and the cementitous structure is very weak when AFt is generated, the volume expansion caused by the AFt generation process will increase the stabilized soil strength; and vice versa.