This paper introduces a method to measure the online strain in the plate shell forming process. It solves the problem that the inside stress online is almost impossible measured in the process of the upper and lower molds forming. The key technology is the design of the moving test frame. This frame can move with the plate’s movement to protect the sensors and ensure the output of the test signal, at the same time, it does not interfere with the shell’s process conditions. It is the first time to apply this method to the large-sized high-strength steel ball shell plate, and it could accurately measure the stress distribution and changes laws within the shell plate under certain specific condition in the process of punching to online stress test and analysis during the punching forming process. The test results show that the periphery of each pressure point is residual tensile stress, and the inside is the residual compressive stress, more evenly distributed. The existence of residual compressive stress is in favor of the anti-fatigue capacity of the plate.