Abstract: Curvic couplings torque transmission with central pull rod preload is a simple and reliable rotor connection structure for aircraft engines. One obvious feature of this connection method is that the axial preload ultimately acts on the end face of small-sized, low-rigidity journal structure, therefore significant preload loss may occur during actual operation. In response to the above issue, force analysis was conducted on the preload process of low-rigidity journal structures, a preload loss model was established, and the actual preload formula was derived to directly obtain the preload loss rate. Taking a certain type of rotor with curvic coupling as the research object, the preload loss of low-rigidity journal structure was analyzed and verified through the numerical simulation and experimental tests. The results showed that the preload loss of the low-rigidity journal structure was about 14% to 18%, which was consistent with the calculated results from the formula of actual preload. It was recommended to fully consider the preload loss and provide necessary preload compensation during the preload design and test stage of low-rigidity journal structure.