The helical tube once through steam generator (SG) of high temperature gas cooled reactors (HTGR) works at very high temperatures. The mechanical performance of the heat transfer tube materials degrades significantly with increasing temperature at its working temperature. The uniformity of heat transfer in the helical tube bundles will influence the integrity and reliability of the SG. Thus, a more detailed analysis of flow and heat transfer uniformity in helical tube bundles is required. Turbulent cross flow and heat transfer in wall bounded (inner cylinder and outer cylinder) helical tube bundles were numerically investigated. Reynolds stress model was used for the turbulence modeling. It was shown that the average flow resistances caused by the helical tubes in the middle passages are the same. The resistance caused by the tubes in the passage near the inner cylinder wall is larger than that in the middle passages. The resistance caused by tubes in the passage near the outer cylinder wall is smaller than that in the middle passages. Considering the influence of the wall effect of the bounding walls, the average velocity and temperature in the passages near the bounding walls are larger than those in the middle passages. Moreover, the average velocity and temperature in the passage near the outer wall are larger than those in the passage near the inner wall. The swing of the turbulent wakes after the tubes near the inner wall are suppressed more severely by the inner wall. Turbulence intensity of the flow passage near the inner wall is smaller than that of the flow passage near the outer wall. Therefore, the heat transfer coefficients of the tubes near the inner wall are lower than those of the tubes near the outer wall.