We investigated the effects of honey bee (Apis mellifera L.) colony size on the pollination of greenhouse-cultivated watermelon grown under the forcing cultivation system. The highest pollination activity of bees was observed (14.3 ± 5.0 honey bees/day) when the bee colony size was 10,000 honey bees, followed by 7,500 and 5,000 honey bees. There was a positive correlation between the bee colony size and pollination activity (R = 0.262). There was no significant difference in the fruit set with different honey bee colony sizes (88%–91%). However, the larger the bee colony size, the higher the number of seeds fertilized and the rate of seed fertilization (p > 0.05). The number of seeds and the content of sugar were negatively correlated (R = -0.714). Overall, we found that a colony size of 5,000 honey bees was the most effective for the pollination of watermelon grown under forcing cultivation.