In order to observe the variation of the expansive soil swelling deformation withupper load and provide the basement for the modelling of swelling deformation， anim- proved testing device was proposed to avoid the compression and variation of the initial dry density because of the upper load in the common loaded swelling test.The comparison tests between the common and the improved were conducted between three kinds of expansive soils.The results show that different upper loads in the common method can occur 1%~3%increase in initial dry density of soil samples， 20%~30%increase in moisture content when the dry density recover to the initial one.In the improved method， the dry density can remain the initialvalue during the whole testing， the loaded swelling ratio obtained by the improved method is smaller than the value obtained by the common method， the regressionparameters used in the predicted swelling deformation are 29%~66%smaller than thatused in the common method.