The tone pattern is consistent in Cheng-Yu group of Southwest Mandarin in Chongqing, but there is an obvious difference about the tone sandhi between Old town Mandarin and Zhongxian Mandarin in Chongqing. The author's opinion: tone sandhi more highlights dialectal feature. The tone system is composed of monosyllabic tone pattern and tone sandhi pattern. "Monosyllabic tone pattern" is the surface form, and "tone sandhi pattern" is the deep form. The monosyllabic tone patterns are identical, and the tone sandhi patterns are different between Old town Mandarin and Zhongxian Mandarin in Chongqing. The tone sandhi patterns of Zhongxian Mandarin reflects the historical relationship between it and the Southwest Mandarin of Hubei province along the Yangtze river.