The leaves of hydroponic lettuce were easy to damage during the process of harvesting in the plant factory, to improve the harvesting quality for hydroponic lettuce, a flexible harvester with low damage was designed and manufactured combined with the agricultural technology of hydroponic lettuce. To realize flexible harvesting with low damage for hydroponic lettuce, image processing was used to detect the outer rectangle of hydroponic lettuce contour, and the plant height and leaf expansion size of lettuce were obtained. The lettuce leaves were grabbed by flexible fingers, and then the lettuce stem was removed by the cutting device. The working process and structure parameters of flexible grabbing device, stem cutting device, and control system were analyzed. The parameters combination of grabbing height ratio, grabbing diametrical ratio, and bending angle for flexible fingers were optimized by using response surface methodology. The optimizing test results showed that the order of significance for harvesting success rate was bending angle, grabbing diametrical ratio, grabbing height ratio, and the single factors had highly significant influence on the leaves damage area. The optimized combination of factors was solved by quadratic fitting function, and the value of grabbing height ratio, grabbing diametrical ratio and bending angle were 0.55, 0.76, 39.7°, respectively. The verification test showed that the leaves damage area was 186mm2, and the harvesting success rate was 96% under the optimized combination, which meant the low damage harvesting for hydroponic lettuce was realized.