Why is the dialect in Qidong County, Hunan Province very similar to Chengdu dialect?

Because Huguang fills Sichuan.

There is a Chengdu song "Zhuzhi Ci" that says: "My aunt married my second aunt Su from Shaanxi, and my eldest sister-in-law was my second sister-in-law Hu from Jiangxi. When relatives and friends first met and asked about their origin, they found that there was no old Chengdu for ten generations." The "old Chengdu" born and raised here is almost No, everyone comes from all over the world and came from the "Huguang fills Sichuan" immigration movement. As we all know, the 34 years of war and plague in the late Ming and early Qing dynasties almost wiped out Sichuan's population. Those bustling towns and fertile farmland were in ruins. The reputation of "Land of Abundance" can only be regarded as a distant memory at this moment. There is a data that best illustrates the dilapidated and withered scene of Sichuan at that time: By the 24th year of Kangxi, the population of Sichuan was still only more than 90,000. Chengdu was uninhabited for thirteen years in the early Qing Dynasty, so the provincial capital could only be temporarily located in Langzhong. This is undoubtedly a tragedy for Chengdu, which has always been the political, economic and cultural center of Sichuan. Since the beginning of the "Huguang Filling Sichuan" mobile movement centered on the Kangxi, Yongzheng, and Qianlong dynasties, Chengdu has gone from dilapidated and decayed to lively and prosperous. Why is this immigration movement called "Huguang fills Sichuan" among the people? That’s because the largest number of immigrants entering Sichuan this time were from Hubei and Hunan (Hubei and Hunan were collectively known as Huguang Province at the time), followed by Shaanxi, Guizhou, Jiangxi, Fujian, Guangdong and other provinces. The immigration movement of Huguang and Sichuan had a profound impact on the modern history of Sichuan. To this day, we can still see the impact of this immigration movement in a modern city like Chengdu.