Abstract: Combined the technics which from convertor,refining,casting to medium plate,the source,shape and distributing level of the non-metallic inclusions in steel plate for boiler and pressure vessel were analyzed. The results showed that the main inclusions were sulfid,silicate,small-grade oxid and sulfid,and little B-type Al2O3. The improvement measures for process control were brought forward,and the effect was satisfying,the reference was offered to develop the high purity steel for boiler and pressure vessel.