In order to solve the issues of hydroponic lettuce manual harvesting with low quality but high cost, a harvester was designed and manufactured. The leaves lying on the hydroponics planting board were gathered by the holding rods before cutting the stem. The key components of the harvester were calculated; for instance, the device of gathering leaves up, the cutter bar and conveyer device. The damage area was measured by using image processing method. The factors those would influence the leaf damage were generalized. The influence of gathering angle and gathering height on damage area was analyzed though the mechanical analysis and highspeed photography. Orthogonal test was utilized to study the influence of gathering velocity, gathering angle, gathering height and velocity of putting lettuce down on damage area. The results showed that the order of significance was gathering angle, gathering height, velocity of putting lettuce down and gathering velocity. The optimized combination of factors that gave the minimum damage area was obtained, and the value of gathering velocity, gathering angle, gathering height and velocity of putting lettuce down were 100mm/s, 15°, 70mm and 100mm/s, respectively. The average value of damage area was 432mm2, and the average value of damage degree was 0.13%, which meant that the optimization was obvious. This research offered a reference for designing the harvester which would harvest more than one hydroponic lettuce.