The invention provides a tension monitoring control device and method for a single-stand reversing mill. The device comprises a coiling machine, a turning roller, the mill, a pulse coder, a thickness gauge, a PLC and a transmission controller, wherein the coiling machine comprises a left coiling machine and a right coiling machine; the turning roller comprises a left turning roller and a right turning roller which are arranged between the left coiling machine and the right coiling machine; the pulse coder comprises a left coder and a right coder which are correspondingly abutted against the left turning roller and the right turning roller; the thickness gauge comprises a left thickness gauge and a right thickness gauge which are abutted against a steel belt moving path between the left turning roller and the right turning roller; a mill opening is abutted against the steel belt moving path between the left thickness gauge and the right thickness gauge; meanwhile, the left coder, the right coder, the left thickness gauge and the right thickness gauge are electrically connected to the PLC; the transmission controller is electrically connected to the PLC, the left coiling machine and the right coiling machine. The device adopting the aforementioned structure can solve the technical problem of self-adaptive adjusting gain compensation and achieves a good effect of accurately controlling the thickness of an ultrathin strip steel.