Energy transfer process and direct charge trapping are two main emission mechanisms in phosphorescent organic light-emitting diodes (PhOLEDs) with host-guest system. Here, to investigate the relationship between luminescence mechanism and performances in host-guest system thoroughly, we demonstrated a green PhOLED using 1,3-bis(carbazole-9-yl) benzene (mCP) as the host and the phosphorescent dye fac-tris(2-phenylpyridine) iridium (fac-Ir(ppy)3) as the guest, where the dopant concentration was variable. It had achieved maximum current efficiency and external quantum efficiency of 84.71 cd/A and 24.5% at 5wt% dopant concentration, respectively. The intrinsic emission mechanisms in the system have been discussed in detail. Moreover, by analyzing the constant current electroluminescence (EL) and the transient EL, the main cause of relationship between dopant concentration and luminescence mechanism has been revealed and the origin of the high performance is also unveiled.