The invention discloses a system for measuring concrete strain distribution in an arch bridge steel tube based on step cross section and a method, belonging to the technical field of concrete stress measurement. The system for measuring concrete strain distribution in the arch bridge steel tube based on step cross section comprises a steel tube and concrete in the steel tube, wherein a plurality of steel bars are fixedly arranged in the steel tube along a radial direction of the steel tube, the neighboring steel bars are provided with a sensor, the sensor comprises a step rod, a sensitive element fixedly arranged on a middle part of the step rod and a protective cover arranged at the outside of the step rod, two ends of the step rod are fixedly provided with end plates; convex parts, far away from the step rod, of the end plates are all connected with the concrete; two ends of the protective cover are respectively in fixed connection with the end plates on the two ends; and a sensor locating piece is also fixedly arranged on the steel bars. According to the system and the method provided by the invention, the sensor comprising different sections of step rods with different cross-sectional area is arranged, thus, a function of increasing sensitivity of the sensor in measurement or expanding measurement range of the sensor is realized.