The double wall plate heat exchanger plate is welded integrally with two thin plate pieces along the corner hole of each pair of plates. When the plate leaks, for example, the perforation caused by long time working will be easily detected from outside. Thereby it stops mixing between two materials.And at the same time, they plays very important roles towards warning internal leakage. Combined with the double seal of the gasket, the possibility of mixing two materials is avoided, and it is suitable for applications with high sanitary requirements or high safety and reliability of industrial production. Since the two sheets are welded to form a sheet, the heat transfer coefficient is inevitably lowered, but the highest safety is achieved in preventing materials mixing.