Abstract: The fillet weld of Q345B steel as an example was welded by E5015 manual arc welding electrode, it was discussed including the macro-segregation between the welded layers, the different of crystalline form and microstructure between layers, and its influence on mechanical properties. The research on microstructure and properties can be advantage to the technical management and welding parameters choice for welding technician. For multi-layer welding, the temperature between layers should be controlled in the underlying layer, the welding quality of products would be ensured by reasonable parameters for field welding.