The invention relates to an on-board dynamic stress testing device and a method. The method is characterized by comprising the steps that 1, runner blades are subjected to inherent frequency testing,and the inherent frequencies of the runner blades are obtained; 2, the on-board dynamic stress testing device for conducting runner blade stress testing is determined and installed in the preset testing point position on the runner blades; 3, an experiment operating point and a test point position for unit stability testing are determined, and a displacement sensor and an acceleration sensor are installed in the preset test point position of a to-be-detected unit; 4, runner blade stress testing and unit stability testing are conducted at the same time, and data are collected synchronously in the testing process; and 5, the obtained rotary blade stress testing data and the unit stability testing data are analyzed, the corresponding relation between the unit stability testing and rotary blade stress testing is obtained, the test result of the inherent frequencies of the runner blades serves as the reference, and the stress of the runner blades are analyzed. The on-board dynamic stress testing device and the method can be widely applied to development and safe stable operation of a water turbine.