The invention provides an extension device for a vehicle lifter platform, capable of increasing the entire length of the vehicle lifter platform by pulling a platform extension moving portion rearward relative to a platform positioning portion. The extension device is so structured that in a platform automatically lifting when driven by a motor or hydraulic pressure, the platform positioning portion and the platform extension moving portion forms the length for normal use, the platform extension moving portion moves of the platform positioning portion towards the platform protruding direction to extend the platform by a desirable length, and the platform positioning portion and the platform extension moving portion are provided with locking mechanisms to lock in a positioning position and an extension moving position.
