The 2.5-terahertz quantum cascade lasers (THz QCLs) based on first-order distributed feedback (DFB) Bragg grating are reported. The loss and threshold gain within different grating duty cycles are calculated according to the mode-coupling theory. The laser output power reaches up to record values of 34.1 mW in pulse mode and 23.4 mW in continuous wave mode at 15 K. The threshold current density is as low as 125  A  /  cm². The single-mode emission is achieved, and the side-mode suppression ratio is 12 dB. In addition, the device is bonded together with the other two DFB THz QCLs (2.9 and 3.0 THz) on a copper chip. The simultaneous different wavelength emissions with single mode are successfully realized.