An operation tool for a fluid injector includes an outer cylinder having a housing extending from a first open end to a second open end, and a selective transfer mechanism provided on the outer cylinder. The selective transfer mechanism moves the fluid injector from an initial position to a first protrusion position and then to a second protrusion position in the housing of the outer cylinder. When the fluid injector is at the initial position, the multi-microneedle device has microneedles retracted from the first open end and positioned inside the housing, when the fluid injector is at the first protrusion position, the microneedles are protruded out to a first distance from the first open end, and when the fluid injector is at the second protrusion position, the microneedles are protruded out to a second distance from the first open end. The second distance is shorter than the first distance.