Abstract: A kind of amended ar ithmetic which solves maximum moment of reinforced concrete isosceles trapezoid slab is introduced. Presently, calculation method of a reinforced concrete isosceles trapezoid slab is experiential arithmetic with which the isosceles trapezoid slab is simplified into an equivalent rectangled slab. It has been verified by analyzing that the relative error of the experiential arithmetic is larger and does not conform to the requirements of engineering design. Based on experiential arithmetic, we use simplified arithmetic of perimeter fixed strut rectangled slab to calculate isosceles trapezoid slab. After amended the error, we acquired satisfactory result. The amended arithmetic features high accuracy, good practicality and convenient calculation.