A theoretical simulation of electrical and optical characteristics of quantum dot (QD) light-emitting diodes depending on the QD sizes is conducted with APSYS software. The electron and hole concentration in the LED and the radioactive recombination rate are studied. Simulation results show that with the increase of the QD size, the emission wavelength has a red shift. With the radius of QD increasing from 1.8 nm to 13 nm , the red shift of emission wavelength has reaches 309.6 meV. The use of the QDs with different sizes planted in quantum well can achieve full-color display with a single LED. When different quantum wells are planted with different QDs, the LED turns into a muti-wavelength luminescence even white LED. We can improve the intensity of each wavelength by adjusting the surface density of QDs. The luminous uniforming of the muti-wavelength LED can be effective improved by adjusting the QD surface density.