A method for thermoforming a bicycle frame component includes the steps of: (A) preparing first and second casings both made of a thermoplastic material, the first casing having a first abutment surface, the second casing having a second abutment surface; (B) preparing a reinforcing unit; (C) abutting the first and second abutment surfaces against each other, and then placing the reinforcing unit at a junction of the first and second casings; and (D) thermally bonding the reinforcing unit to the first and second casings such that the first and second casings are joined to form the bicycle frame component. A bicycle frame component is also disclosed.