A liftable keyboard for a portable electronic device is provided. The portable electronic device includes a top cover and a base. The liftable keyboard includes a key unit, a membrane switch circuit layer, a vent valve and a pump. The key unit includes a keycap and an elastomer. The vent valve includes an inlet, an outlet and a gas exhaust chamber. The inlet and the outlet are in communication with a gas-inputting port and a gas-outputting port of the membrane switch circuit layer, respectively. The gas exhaust chamber is in communication with the pump. When a sensing element of the base senses that the top cover is close to the base, the pump is enabled to perform a gas-extracting action. The elastomer is subjected to compressed deformation and the keycap is moved downwardly. Consequently, an altitude of the key unit is reduced.