To overcome the shortcomings of the traditional hot-water boilers, a oil-fired hot water boilers is improved design for a group-annulus structure. A finished product boiler is thermodynamic analyzed with the tested heat generation amount equivalent to 350 kW. The results indicate that the unique heat exchange structure of group-annulus type hot-water boilers can reduce thermal stresses and intensify convection heat exchange. The radiation heat-exchange space is 2 times more than that of the shell-tube boiler, and surfaces inside the furnace are comparatively large. Compared with shell-tube type boilers, the group-annulus type boilers is made a distinction with small volume and heat-exchange surface area, low metal consumption and high ratio of performance to cost under the same total heat-exchange quantity.