To improve wear resistance and service life of boron cast iron cylinder, plasma beam hardening on surface is adopted.Scanning electron microscope,X-ray diffractometer and micro-hardness tester is used to analyze the effect of working current on the structure and properties of plasma beam hardening layer at the junction of the hardening traces. The results show that the microstructure of hardening layer at the junction is hidden acicular martensite, retained austenite and flake graphite. With the increase of working current, the content of retained austenite decreases, the hardness and depth of the hardening layer increases. The highest hardness is not achieved at the surface of hardening layer but at the second-surface layer which has a certain distance to the surface. The uneven distribution of hardness in hardening layer leads to large gradient at the both sides of the highest hardness and the gradient decreases with the increase of working current.