To balance the carrier injection in Quantum Dot Light Emitting Diode (QLEDs), a method to determine the thickness of each functional layer was proposed. The thickness of EL was set firstly, the thickness of Electron Transport Layer (ETL) was determined through tunneling model. On this basis, the thickness of Hole Injection Layer (HTL) was determined at last using space charge limited current model. Devices using red light CdSe/ZnS quantum dot was used as the Light-Emitting Layer (EL), poly-TPD as the HTL, Alq3 as the ETL were fabricated through spin-coating and vacuum evaporation with the thickness of each layer obtained from the simulation results. Experiment results show that compared with other devices, the device with a 45 nm thickness of poly-TPD, 25 nm QDs and 35 nm Alq3 has a higher luminous efficiency and color purity. QLEDs with the thickness of each layer chosen according to the method proposed have a more balanced carrier injection which is advantageous in improving device performance.