The invention discloses a portable determining device and a portable determining method for corrosion rate of a steel bar. The portable determining device comprises a steel bar fixing system, a steel bar heating system, a temperature control system and a thermal imaging scanning system, wherein the steel bar fixing system comprises a base and a steel bar sleeve; the steel bar sleeve is fixed on the base; a steel bar plug is arranged in the steel bar sleeve; the steel bar heating system comprises a heating device I and a protecting cover; the heating device I is arranged on the inner wall of the steel bar sleeve; the protecting cover is arranged outside the first heating device; the surface of the first heating device is covered with an insulating layer; the temperature control system comprises a temperature sensing strip, a temperature control switch and a temperature recorder; the temperature sensing strip is locally embedded in the insulating layer I; the temperature control switch and the temperature recorder are connected with the temperature sensing strip; the thermal imaging scanning system comprises a heat-insulating sleeve arranged on the exterior of the steel bar sleeve in a sleeving manner, a bar-shaped thermal imaging instrument arranged on the inner wall of the steel bar sleeve and a computer connected with the thermal imaging instrument. The portable determining device disclosed by the invention adopts a thermal imaging technique for determining the corrosion rate of the steel bar, is detachable and portable, is high in determining precision and is capable of directly reflecting the corrosion form of the steel bar and the variation trend of the corrosion rate.