Abstract: The luminous efficiency and energy consumption of LED (light emitting diode) are affected by heat dissipation of high power LED. In the study, LED surface light source used in plant factory with water-cooled cooling system was designed, and the purpose was to take away the heat generated by the LED using water cycle, to reduce the junction temperature and delay light attenuation of the LED, to distribute the heat according to requirement for heat of plant factory, and thus to reduce energy consumption for controlling temperature. The system was mainly made of LED surface light source, radiator and water circulation system. LED surface light source was made of the high-power LED, with 28 identical array units which were arrayed in 4 rows and 7 columns, and each array unit consisted of 44 LEDs. LED surface light source could give out red, blue, white, green and yellow light or any combination light made by above-mentioned light. Supplying voltage of light source was 220 V, and power of LED was provided by the conversion from 220V AC (alternating current) signal to 0-48V DC (direct current) signal. Electrical energy of the light with different colors was respectively supplied by independent power. Dimmer was used to adjust output DC voltage of the power, to regulate current of the LED and realize the aim of adjusting light intensity. Light intensity of LED surface light source could be adjusted in the range of 0-2000 μmol/(m2·s). The radiator of LED surface light source (length×width×height, 500 mm×400 mm×20 mm) was made of hot galvanized Q235 steel, which was composed of bottom plate, hanging lug, cover plate, etc. Water path of bottom plate was S type. The function of water circulation system was to transfer the heat produced by LED surface light source to water. After giving out heat, water was recycled into the radiator to maintain the temperature of the radiator at a low level. The water circulation system mainly consisted of supplying water tank, constant-flow pump, recycling water tank, valves, radiators, sensors and pipelines, etc. The waterway was divided into 2 parts; in the heating season, the heat was input into plant factory for raising indoor temperature, while in the non heating season, the heat was output to the outdoor, and then water was cooled by air before entering the water circulation system. During the time from 2 to 6 a.m. of November 2, 3 and 4, 2014, the effect of the cooling system of LED surface light source was tested in 13 m2 plant factory in the Life Science Building of Nanjing Agricultural University. Under water cooled and non-water cooled conditions, radiator temperatures of 4 corners on the radiator were measured, and average value for each measurement was recorded. The results showed that water-cooled system had obvious effect of reducing temperature of LED surface light source. Compared with non-water cooled system, plate temperature was lower, which was decreased by at least 5.3℃ and up to 19.3℃. Water-cooled system directly reduces the temperature of the radiator, and indirectly reduces the junction temperature of the LED. And thus it can prolong the service life of LED surface light source.