Blast walls are widely used to prevent important architecture from terrorist attacks and accidental explosion. Diffraction flow effect makes the explosive shock flow field change and overpressure at a certain distance behind the wall is greatly increased. LS-DYNA was employed to study the diffraction flow and overpressure time-history behind blast walls in the case of an explosion near above ground. It is observed that the diffraction flow overpressure behind a wall about 1. 5 ~ 2. 5 times wall height is large,and it is more dangerous than there is no wall owing to Mach reaction happened around double wall height behind the wall. The research provides a reference for arrangement of blast walls and blast-resistant design of buildings behind the blast wall.