Abstract: An extension ladder comprises a plurality of ladder sections, connection kits and transverse step beams. Each ladder section is a hollow rail, which can be interconnected by telescoping with each other. A snap-fit mechanism is provided between the upper ladder section and the lower ladder section to make them not slip when the ladder is extended. The upper end of each ladder section is provided with a connection kit. The upper end of the connection kit is provided with a fixed protuberant pin, an inside thereof is provided with a locking mechanism which is provided with a turning key. An outside of the connection kit is covered by the transverse step beam. When the upper and lower connection kits are close to each other, the protuberant pin of the lower connection kit pushes the turning key so that the locking mechanism of the upper connection kit can be unlocked.