Abstract: In order to identify the reasons for the low and unstable qualified rate of flaw detection of 800 MPa hydroelectric steel plates, in this study, samples were taken from the steel plates that did not meet the flaw detection requirements after rolling, and the samples were detected by optical microscope, scanning electron microscope and energy spectrometer. Research shows that internal cracks in steel plates are the direct cause of non-conformity in flaw detection. The length of the cracks increases with the increase of the grade of Class B inclusions in the steel plates. The length of the cracks in the steel plates rolled from the continuous casting billets of the first and last castings is twice that of the middle castings. Severe segregation and enrichment phenomena of P, S and microalloying elements occurred inside the steel plates that did not meet the flaw detection requirements. On this basis, the smelting process and steel grade composition of 800 MPa hydroelectric steel were optimized and controlled, increasing the qualified rate of steel plate flaw detection from 95.6% to 98.6%, and stabilizing the fluctuation range from 92.5%-98.8% to 96.8%-99.5%.