Abstract: Thin rod bundles with tight lattice are arranged according to the equilateral triangle grid. As the proportion of fuel is large, the power density of core is high. Based on the analysis of the capability of core, the ABV-6M reactor was taken as the example, and two objective functions, power density and flow rate of coolant were proposed for optimization calculation. Diameter and pitch of rods were optimized using the genetic algorithm method separately. The result shows that tight lattice is effective for improving the performance of the reactor core and the core power density compared with the core parameters in literature.