An extension ladder, including multiple ladder sections, each including a pair of pin holes, a pair of columns, and a rung, including a pair of pins, a pair of springs, a gear, a pair of gear racks, and a pair of mounting rings, wherein the columns of adjacent ladder sections are fit with each other, diameters of the columns of the ladder sections increase from the top to the bottom, the pin holes are disposed on the columns, and operate to receive the pins, the pins are coaxial, the gear is disposed on a side wall of the rung and engaged with the gear rack, one end surface of the gear sticks out from the side wall of the rung, a key hole is disposed on the end surface of the gear, the mounting ring operates to fix the pin, and the spring is contacted with the pin.