Fusion plasma density can be measured by FIR laser and submillimeter wave interferometer. The DCN laser has the highest specific output power among known submillimeter electricdischarge lasers. Its output characteristics fit the requirements for interferometric measurements on large tokamaks. So that, the continuous wave (CW) discharge-pumped 1.54 THz (195 mum) and 1.58 THz (190 mum) deuterium cyanide (DCN) laser has been designed and developed as the source of FIR laser diagnostics in the EAST tokamak. The laser cavity is a 54 mm inner diameter, 3 m long of pyrex tube with plane reflectors (one of them is a nickel mesh) against its ends. A hot LaB6 cathode is essential in producing a stable discharge at high current density.