The organic phosphorescent OLED (PhOLED) has been widely studied because its inner quantum efficiency can reach 100%, but there has been much debate about the internal luminescence mechanism and process, mainly because they are explained using the luminescence theory of inorganic LED. In this paper, we set up a transient electroluminescence (EL) and delay EL measurement system, and for the first time asfar as we know use this measurement system to study the internal luminescence mechanism and process of PhOLED. In these studies we first fabricate a PhOLED which uses a kind of new efficient red-emitting iridium(III) complexes (Bis[2-(9, 9-dimethyl-9H-flouren-2-yl) benzothiazolato-N, C2'] iridium(III) (acetylacetonate)) doped with TAZ as the emitting layer. From the results, we find that there exists an overshoot at the end edge of the driving pulse; through the research we find that this is reasonable for the holes and electrons to accumulate in the object materials Irf and host TAZ, respectively. We also find that at the interface between host transfer layer and emission layer there exist a large number of holes. Through the delay luminescence measurement, we have proved that the emission of this doped system mainly comes from the directly trapped holes and electrons in Irf, and then excitons are formed.