In this paper, the laminated structure of quantum dot (QD) light emitting devices were made by using CdSSe/ZnS quantum dots with diameter of about 10nm as the light emitting layer. The influence of the quantum dot layer thickness on the film morphology and the performance of the quantum dot light emitting diodes was investigated. Atomic force microscopy (AFM) results showed that the quantum dots were agglomerated when the quantum dot layer was too thick, and the aggregation phenomenon was weakened with the decrease of thickness. When the quantum dot layer thickness and quantum dot size were equivalent (about 10nm), the quantum dots were arranged in a single layer and the aggregation phenomenon disappeared. When the thickness of the quantum dot layer was below 10nm, the film was defective with holes. The experimental results showed that the thickness of quantum dot layer in QD LED was closely related to the photoelectric properties of the devices. The optoelectronic properties of the device with the quantum dot layer thickness of 10nm were the best. It had the lowest light-up voltage of 4.2V, the highest brightness of 446cd/m2 and the highest current efficiency of 0.2cd/A. The method of changing the thickness of the quantum dot layer by controlling spin speed was simple and reproducible, and had a certain application value to QD-LED research.