Guest/host phosphorescence materials have attracted much attention; traditionally, researchers have focused on the influence of the electronic properties and energy levels of the molecules on the phosphorescence activities. However, the effects of the morphology on the phosphorescence properties are ignored. Herein, three isoquinoline guests with different aliphatic rings and three hosts are selected to construct guest/host materials. Experimental results confirm that the guests are dispersed in the host in the form of clusters. More importantly, the morphologies of the guest/host directly affect the phosphorescence properties. In these systems, the guests have strong intermolecular interactions, which are beneficial to stabilize the triplet excitons; meanwhile, the hosts should have weak intermolecular interactions with easily changed morphology to accept the guest clusters, which synergistically ensure that the doped materials have excellent RTP properties. This is the first work focusing on the effect of molecular morphology on the phosphorescence characteristics of guest/host systems.