A method of assembling an airtight LED light bulb has the steps of: connecting a stem device with an LED device, drying the LED device, connecting the stem device with a bulb envelope, extracting air in the bulb envelope via a pipe, filling the bulb envelope with nitrogen or inert gas via the pipe, sealing an opening of the pipe which is located outside the bulb envelope to make the bulb envelope completely airtight and connecting a cap with the bulb envelope. Because the bulb envelope is airtight, moisture in the environment can not damage the LED device, and the steps of extracting air in the bulb envelope via the pipe and filling the bulb envelope with nitrogen or inert gas via the pipe are feasible. Consequently, the LED device will not easily be oxidized or dampened, so the lifespan of the airtight LED light bulb can be prolonged.