The microcavity organic light-emittin g device (OLED) was fabricated with highly reflective Al cathode and the semitransparent Al anode.The device s tructure is as follows:Al(10nm)/MoOx(Y nm)/NPB(40n m)/TCTA(10nm) /CBP:R-4B (4%)(30nm)/BCP(10nm)/AlQ(40nm)/LiF(1nm)/Al(100nm),where R-4B i s empl oyed as red phosphor emitting layer.The effect of cavity length on OLED propert y is discussed in this paper.It′s indicated that the spectrum is narrowed and the color purity is increased.With the increase of MoOx thickness,the light-e mitting peak changes from 600nm to 668nm.The optimal electroluminescent perfor mance is observed in the sample with the MoOx thickne ss of 40nm:the emitting pe ak wavelength is 608nm,the full width at half maximum is 50nm,the maximum br ightness reaches 35300cd/cm²,and the maximum current effic iency is 23.5cd/A.Red organic phosphor electroluminescent devices with stable luminescent properties a re obtained.