Charge generation unit consisting of B₃PyMPM:Cs/Al/HAT-CN was employed to fabricate a highly-efficient tandem green phosphorescent organic light-emitting device. The maximum current efficiency and the maximum luminous efficiency of tandem organic light-emitting device are as high as 172.2 cd/A and 111.0 lm/W, respectively. At the given current density of 5 mA/cm², the voltage and luminance of the tandem device are 2.04 times and 2.84 times those of single-unit device. In order to explore the reason why the performance of tandem device is better than the single device, the charge generation and injection processes in charge generation unit, and the effect of ultrathin Al layer on electron injection capability and the stability of charge generation unit systemically were studied. Experimental results show that the charges can be efficiently generated in the charge generation unit and smoothly injected into the electron transport layer. The insertion of the thin layer of aluminum between B₃PyMPM:Cs and HAT-CN can increase both the electron injection and the device stability.