Abstract: An experimental investigation on longtime cooling is carried out in a removal of all core power condition for understanding the running characteristics of ACP100 passive residual heat removal system（PRS）.The investigation finds a continuance of natural circulation,which is due to stored heat releasing from reactor,core and steam generator.The natural circuit flow of PRS keeps from 0.52 t/h to 0.26 t/h;meanwhile,the system pressure of PRS falls from 1.0 MPa to 0.51 MPa,and the outlet temperature of the reactor core falls from 178.1℃ to 105.0℃.Besides,the investigation shows that natural circulation is descending continuously,and never restarts after stagnation.