A liftable screen stand, characterized in that the liftable screen stand comprises: a base assembly; an elevating device mounted on the base assembly and having a guiding tube being hollow, mounted on the base assembly, and having an interior; and an outer surface; an elevating tube mounted to the guiding tube and being movable along the guiding tube; a supporting tube being hollow, mounted on the base assembly, disposed in the interior of the guiding tube, and having an interior; and a cover mounted at a top of the supporting tube; a pedal seat mounted in a lower half of the base assembly; a pedal pivotably mounted on the pedal seat and having a passive portion disposed at an end of the pedal and disposed below the supporting tube; and a pedalled portion connected to the passive portion and located at an exterior of the base assembly; a pushing rod disposed in the interior of the supporting tube, passing through the base assembly, and abutting against the passive portion of the pedal; and a pneumatic cylinder mounted to an inner top surface of the elevating tube, disposed in the interior of the guiding tube, and having a gas spring fixed at the cover of the supporting tube; an activating valve mounted at a bottom of the gas spring; and an elevatable tube movably mounted to the gas spring and having a top end fixed on the inner top surface of the elevating tube; and an assembling device for supporting a screen mounted to the elevating tube of the elevating device and being at a top of the elevating tube; wherein to lift the screen stand, the pedaled portion is pushed in a downward direction for the passive portion to push the pushing rod upwardly, and the pushing rod then pushes the activating valve of the pneumatic cylinder to move the elevating tube upwardly for rising the assembling device to a higher position. a base assembly; an elevating device mounted on the base assembly and having a guiding tube being hollow, mounted on the base assembly, and having an interior; and an outer surface; an elevating tube mounted to the guiding tube and being movable along the guiding tube; a supporting tube being hollow, mounted on the base assembly, disposed in the interior of the guiding tube, and having an interior; and a cover mounted at a top of the supporting tube; a pedal seat mounted in a lower half of the base assembly; a pedal pivotably mounted on the pedal seat and having a passive portion disposed at an end of the pedal and disposed below the supporting tube; and a pedalled portion connected to the passive portion and located at an exterior of the base assembly; a pushing rod disposed in the interior of the supporting tube, passing through the base assembly, and abutting against the passive portion of the pedal; and a pneumatic cylinder mounted to an inner top surface of the elevating tube, disposed in the interior of the guiding tube, and having a gas spring fixed at the cover of the supporting tube; an activating valve mounted at a bottom of the gas spring; and an elevatable tube movably mounted to the gas spring and having a top end fixed on the inner top surface of the elevating tube; and a guiding tube being hollow, mounted on the base assembly, and having an interior; and an outer surface; an interior; and an outer surface; an elevating tube mounted to the guiding tube and being movable along the guiding tube; a supporting tube being hollow, mounted on the base assembly, disposed in the interior of the guiding tube, and having an interior; and a cover mounted at a top of the supporting tube; an interior; and a cover mounted at a top of the supporting tube; a pedal seat mounted in a lower half of the base assembly; a pedal pivotably mounted on the pedal seat and having a passive portion disposed at an end of the pedal and disposed below the supporting tube; and a pedalled portion connected to the passive portion and located at an exterior of the base assembly; a passive portion disposed at an end of the pedal and disposed below the supporting tube; and a pedalled portion connected to the passive portion and located at an exterior of the base assembly; a pushing rod disposed in the interior of the supporting tube, passing through the base assembly, and abutting against the passive portion of the pedal; and a pneumatic cylinder mounted to an inner top surface of the elevating tube, disposed in the interior of the guiding tube, and having a gas spring fixed at the cover of the supporting tube; an activating valve mounted at a bottom of the gas spring; and an elevatable tube movably mounted to the gas spring and having a top end fixed on the inner top surface of the elevating tube; and a gas spring fixed at the cover of the supporting tube; an activating valve mounted at a bottom of the gas spring; and an elevatable tube movably mounted to the gas spring and having a top end fixed on the inner top surface of the elevating tube; and an assembling device for supporting a screen mounted to the elevating tube of the elevating device and being at a top of the elevating tube; wherein to lift the screen stand, the pedaled portion is pushed in a downward direction for the passive portion to push the pushing rod upwardly, and the pushing rod then pushes the activating valve of the pneumatic cylinder to move the elevating tube upwardly for rising the assembling device to a higher position.