A steam generator (1) includes a heat exchanger (13), a liquid header (11) and a steam header (12). The heat exchanger (13) is assembled by several heat exchanging subassemblies (2) with the same structure. The heat exchanging subassembly (2) includes a spiral heat transmission pipe bundle (3), a central cylinder (4) and a sleeve (5). The spiral heat transmission pipes with different radii are concentrically and spirally arranged in annular space between the central cylinder (4) and the sleeve (5) to form one or more concentric heat exchanging pillar surfaces (6). One end of the liquid header (11) is connected with a main water feeding pipe (14), and the other end of the liquid header (11) is connected with the spiral heat transmission pipe bundle (3). One end of the steam header (12) is connected with a main steam pipe (15), and the other end of the steam header (12) is connected with the spiral heat transmission pipe bundle (3).