The invention provides an internal stress measuring method of an opening plate, belonging to the technical field of measurement. An opening plate is evenly cut into strips via marking benchmark straight lines on the opening plate for releasing the internal stress; the bending deformation of the benchmark straight line, corresponding to each measuring strip, is measured; the internal stress beforereleasing the stress is calculated according to the bending deformation of each measuring strip, so that the internal stress distribution of the whole opening plate is obtained. Advantageously, underthe precondition of satisfying the internal stress measurement precision of the opening plate, sample preparation and measurement can be performed by common workers with conventional cutting tools, ordinary rulers and tapered rulers; and the internal stress of the opening plate is obtained fast by a mathematical model built by the invention. The method of the invention is used in the works of theprocess optimization of hot-rolling ship plates and opening process optimization of chip plates, and respectively used for measuring the internal stresses of D36 and D32 levels of ship plate opening plates which are 2000 mm in length and 2000 mm in width, so that the internal stress distribution of the ship plate opening plate is obtained accurately, and reliable reference data is provided for the optimization of production process.