Organic semiconductor (OSC) devices based on manipulation of electron spin have attracted considerable attention since the discovery of long spin relaxation time and large transport distance in OSCs. For organic light-emitting devices (OLEDs), controlling the singlet to triplet ratio by spin-polarized electrodes is one of the effective ways to realize high luminescent efficiency. Based on the drift-diffusion equation, continuous equation and Langevin recombination theory, the spin injection, transportation and recombination properties of carriers in OLEDs are modeled in this paper. The density of polarized electrons and holes in OSCs are calculated, the singlet to triplet ratio is analyzed, and the influences of the electrical field, spin-related interfacial conductance, bulk conductivity and polarization of electrodes are accounted for. It is showed that opposite spin polarization of electrons and holes are in favor of increasing singlet to triplet ratio, and the higher spin polarization of injected carrier density is, the larger singlet to triplet ratio will be. Large spin-related interfacial resistance, large polarization of electrodes, matched bulk conductivity and high electrical field under forward bias favor spin polarization of carries density in OSCs. We can obtain obviously improved density polarization by optimizing the related parameters on the basis of essential injection efficiency. The optimized polarization ensures sufficient space for manipulating singlet to triplet ratio, hence the quantum efficiency of OLEDs.