A lambdoidal ladder comprises front struts (100), back struts (200), treads (500), a folding device) 400, a connection device (600) and a platform (300). Wherein the platform (300) is connected to the front strut (100) and the back strut (200), and the structure between the front strut and the back strut is a rotatable connection; the platform (300) comprises a body (310) and a transverse rod (320) which goes through the front strut (100), an orientation slot (314) matching to the transverse rod (320) is set under the body (310); the folding device (400) fixed to the middle part of the front strut and the back strut is constructed a hinge with a H figuration; the tread (500) is fixed to the front and back struts by the connection device (600).