The utilization of a double emitting layer structure with well-matched energy levels has been shown to improve carrier balance and decrease operating voltages in organic light-emitting diodes (OLED). In this study, tandem OLEDs with double emitting layers (DEMLs) are successfully fabricated and optimized. Through adjusting the emitting layer thickness ratio and cavity length, the optimized green tandem OLEDs show driving voltages of 4.8 and 5.4 V, with current efficiency (CE)/power efficiency (PE) of 242.4 cd/A/158.6 lm W−1 and 237.1 cd/A/137.9 lm W−1 at 100 and 1000 cd m−2, respectively. The ultra-high CE of 242.4 cd/A is the state of the art among all reported two-units green tandem OLEDs. Meanwhile, the device at the initial luminance of 10000 cd cm−2 also displays an extremely long LT50 of 734.8 h. In addition, the tandem OLED can emit pure green light with color coordinates of (0.27, 0.71), and has the ability to adjust the color coordinates by varying the cavity length.