The flow passage of a FUNKE"FPG" semi-welded plate heat exchanger is composed of a welded flow passage and a routine flow passage in a crosswise way. When processing medium corrosive to the sealing gasket, the medium can flow through the welded flow passage. Welded flow passage and routine flow passage are formed in the following ways: weld the whole sealing perimeter of two corrugated sheet bars with laser to be "sheet bar pair" to form welded flow passage. The space between each pair of sheet bars shall be sealed with gaskets, forming removable routine flow passage. Semi-welded plate heat exchanger is mainly applicable to the condenser and evaporator of ammonia refrigerating system as well as heating and cooling of solvents.