The microchannel rib structure was optimized to improve thermal performance, temperature uniformity, and flow field homogeneity. The realizable k-ε turbulence model was employed to compare and discuss the cooling performance of a fully enclosed channel and four non-enclosed channels with varying fin heights. Model accuracy was validated experimentally. Results showed that adopting non-enclosed configurations effectively improved flow field homogeneity, resulting in a uniform velocity distribution. An optimal fin height was identified during the transition from the enclosed channel to decreasing fin heights, maximizing both the temperature uniformity and thermal performance of the heat sink. Under identical conditions, among the five configurations studied, the microchannel heat sink with a fin height of 1.8 mm exhibited the best thermal and temperature uniformity performance. Compared to the enclosed channel design, the average temperature was reduced by 4.22% and the maximum temperature difference was reduced by 7.4%. Compared to the design with a fin height of 1.2 mm, the average temperature was reduced by 14.95% and the maximum temperature difference was reduced by 15.43%.