We present a simple and accurate optical model for simulation of the optical properties of top-emitting organic light emitting diodes (OLEDs). Based on the model, we investigated the optical properties of top-emitting OLEDs and optimized the structure of a blue top-emitting OLED based on 4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl. The device optimization process focused on matching the optical properties of each functional layer. The out-coupling efficiency of device with optimized structure can be enhanced by 80%; in addition, the color purity of blue emission is improved. Experimental results show excellent agreement with the simulation results.