A pressure tank includes a metallic vessel, a plastic liner received in the metallic vessel, a flexible diaphragm, two metallic joints, two anti-leak assemblies and a nozzle coupled to the metallic joints respectively. The metallic vessel includes upper and lower shells. The upper shell defines a first planar area on a side thereof and a second planar area on a top thereof. The lower shell defines a third planar area therebottom. The flexible diaphragm divides said metallic vessel into a storage space and a pneumatic room. The metallic joints are mounted on the side and top of the upper shell respectively and are in communication with the storage space. The two anti-leak assemblies provide leakproof connection between the metallic joints and the plastic liner. Additionally, the nozzle is mounted on the third planar area to be in communication with the pneumatic room.