A novel coupled barrier heterojunction structure (i.e. Al0.26Ga0.74N/GaN/Al0.20Ga0.80N/GaN) was proposed and prepared in this paper to improve transconductance linearity of GaN high electron mobility transistor. A double two-dimensional electron gas (2DEG) channel in the coupled barrier heterojunction structure is demonstrated by theoretically calculation and capacitance-voltage (C-V) measurement. 2DEG mobility of 1 510 cm2·V-1·s-1 and sheet density of 9.7×1012 cm-2 are obtained. Benefiting from the double channels, the transconductance profile of the device with coupled heterojunction barrier structure shows two peaks, and the voltage swing is 3.0 V, which is 1.5 times larger than that of normal structure. First-order and second-order derivative characteristics of transconductance indicate that the device of the coupled barrier structure has higher harmonic suppression capability. This study demonstrates a superiority of coupled barrier heterojunction in the area of high linearity application.