Abstract: The mechanical properties of concrete-filled steel tubes (CFST), including elastic modulus and load-bearing capacity etc, and the effect of pouring pulp were investigated experimentally under the condition of separation of concrete and steel tube. The result indicates that with the increase of separation width, the elastic modulus and load-bearing capacity of a CFST decrease greatly, and its longitudinal and lateral deformations increase sharply. The reason for this is that when the separation of concrete and steel tube happens, they can not bear loads together. After pulp is poured for a separated CFST, it can renew its original mechanical properties basically.