A could manufacturing service resource optimization under group buying mode was proposed in order to reduce service costs of demanders. In the early stage of cloud manufacturing platform, service resource optimization management was analyzed from perspective of service demanders, so as to reduce their service costs of cloud manufacturing platform as a whole. The group buying mode was introduced into the resource combination optimization model, and the key impact factors including pricing and trust level in group buying were considered. Under group buying circumstances, comprehensive decision of cloud manufacturing resource optimization was studied, and an improved genetic algorithm was designed for solving this model. Furthermore, the simulation experiments of different scale problems were also given to verify the validity and feasibility of the proposed model and the improved algorithm. The simulation results show that, when group buying scale increases, group buying mode has more cost advantages.