The utility model discloses an extensible steel truss joint assembling device comprising a movable lift platform and an operating platform disposed on the top of the movable lift platform. The extensible steel truss is characterized in that the operating platform comprises a fixed platform which is fixedly connected to the top of the movable lift platform and is of a hollow structure; at least one extensible platform is arranged inside the fixed plate; rollers are uniformly arranged on two opposite sides of each extensible platform; a plurality of limit blocks are arranged between the extensible platforms and the fixed platform, and the fixed platform is further provided with a power mechanism used for controlling rolling of the rollers. The extensible steel truss joint assembling device is safe and reliable, convenient to use and capable of protecting construction staff better and reducing workload thereof.