Longmen County, under the jurisdiction of Huizhou City, Guangdong Province, was originally the Shanglongmen area of Zengcheng County, so it was named Longmen. It is located in the middle of Guangdong Province, at the upstream of Zengjiang River, bordering Heyuan City and Boluo County, Huizhou City in the southeast, Conghua District and Zengcheng District, Guangzhou City in the southwest, and Xinfeng County, Shaoguan City in the north, with a total area of 2267.2 square kilometers. By 2019, Longmen County had jurisdiction over 2 streets, 7 towns and 1 Yao township. According to the data of the seventh population census, as of 0:00 on November 1, 2020, the permanent population of Longmen County is 319183.

In the 24th year of Qin Shihuang (223 BC), Panyu County was set up, and Longmen belonged to Panyu County. From the founding of the county to the end of the Qing Dynasty, Longmen County belonged to Guangzhou. In March 1959, it belonged to Foshan. In January 1975, it was subordinate to Guangzhou; Since April 1988, Longmen County has been under the jurisdiction of Huizhou City. In November 2017, Longmen County was awarded the title of "National Civilized County".

The main body of Longmen culture is Guangfu culture, whose essence is greatly influenced by marine civilization. It is bold to be the first, inclusive and tolerant. Longmen County is the intersection of Guangfu culture, Hakka culture and Yao culture, and is known as the "Ecological Garden of the Pearl River Delta" and "Oasis on the Tropic of Cancer".

In 2021, Longmen County will achieve a gross regional product (GDP) of 18.787 billion yuan.