This experiment mainly monitored the high-density planting conditions of small watermelons in spring greenhouses in Beijing by US350 environmental sensors. Besides, it analyzed the requirements for environmental factors in different growth stages of watermelons. The experiment obtained the following results. (i) Under the condition of high-density planting of small watermelons, the average air temperature range was 17.48-21.16℃ at the seedling hardening stage, the average air temperature range was 16.68-25.53℃ at the tendril elongation stage, and the average air temperature range was 15.98-32.06℃ at the fruiting stage. The average soil temperature range was 20.56-23.03℃ at the seedling hardening stage, the average soil temperature range was 20.44-25.51℃ at the tendril elongation stage, and the average soil temperature range was 18.72-28.84℃ at the fruiting stage. The average light intensity range was 2 359.15-8 207.44 Lux at the seedling hardening stage, the average light intensity range was 551.98-9 170.77 Lux at the tendril elongation stage, and the average light intensity range was 1 253.59-9 644.01 Lux at the fruiting stage. The average relative humidity range was 60.69%-76.40% at the seedling hardening stage, the average relative humidity range was 60.85%-89.89% at the tendril elongation stage, and the average relative humidity range was 42.14%-93.00% at the fruiting stage. The CO2 concentration range was 455-631 mL/m3 at the growth stage.