The invention discloses a bike saddle tube assembly with an anti-theft structure, which includes a saddle tube, a fixing seat positioned at the upper end of the saddle tube, a connecting piece positioned at the lower end of the saddle tube, and a bolt connecting the fixing seat with the connecting piece, wherein a through hole enabling a screw of the bolt to penetrate through is formed in the fixing seat; the diameter of the through hole is slightly larger than the outer diameter of the screw of the bolt; the fixing seat is lapped with the upper end of the saddle tube; the connecting piece with the diameter not larger than the outer diameter of the saddle tube is arranged at the bottom end of the saddle tube; a screw hole with internal threads and a positioning bulge positioned inside the saddle tube and near to the inner wall of the saddle tube are eccentrically arranged on the connecting piece; the screw of the bolt penetrates through the fixing seat and is connected with eccentric threads of the screw hole of the connecting piece at the bottom of the saddle tube. The bike saddle tube assembly enables the saddle tube with an eccentric piece to be tightly clamped with a middle tube of the bike frame, and effectively solves the anti-theft problem that the conventional clamping ring with an eccentric quick-dismounting part locks a bike saddle rod as the saddle tube can be taken out of a middle tube only through a tool.