We have designed and fabricated a traditional organic light-emitting diodes(OLED).We appended a kind of hole blocking material(BCP) in the device and altered the thickness of BCP and adjusted the balance of the electron and hole to control the light-emitting zone.It is shown in the experiments that when the BCP thickness is 0 nm,the device structure is typical double layer organic electroluminescent structure and the character spectrum is green emitting of Alq_3;when the thickness is 8 nm or over 8 nm,the device is blue emitting of NPB;when it is between 1 nm and 8 nm,NPB and Alq_3 both contribute to emitting,and EL spectrum includes the blue and green emission.BCP layer plas the role to regulate carrier compound area and change color.So the device performance will be improved by controlling the BCP thickness.