The transient characteristics associated with the machine load rejection process are investigated due to its close relationship with hydraulic facilities' safety in low-head tubular turbines, especially in double-regulated bulb turbine. Load-rejection transient characteristics of a double regulated bulb turbine during the co-closing guide vanes (GVs) and runner blades (RBs) procedure were numerically and experimentally studied in this paper. The co-adjusting of GVs and RBs is realized through the usage of dynamic mesh technology. During the process, the speed is augmented by 33.9% by the time it reaches its peak value, where the corresponding maximum negative axial force is 43% of the initial value. The three-dimensional (3D) unsteady numerical simulation method reveals that severe impact of backflow from RBs causes significant fluctuation of torque and axial force during the co-closing procedure. A low-pressure vortex rope is also cut off by gravity-dominant pressure distribution, which leads to hydraulic instability and complex flow structure. The system performance characteristics have been obtained through experimental testing, where the dynamic instability during the co-closing procedure has been showcased. This article presents a novel prediction model and a foundations for further transient characteristics studies and prediction for turbine system with adjustable RBs and GVs. (C) 2020 Elsevier Ltd. All rights reserved.y