Abstract: In the middle ages of ancient China, the entering and returning tones were the main phonetic features of Renfu Dialect of Southwest Mandarin. Such dialect distributed mainly in the geographic areas of Neijiang and Zigong which are now between the lower reaches of Min and Tuo Rivers. Such phonetic features of the dialect are stable. They were brought about by the internal factor that the same type of tones(with similar pitches)fused into each other and the external one that the local dialect contacted with the migrant one and was influenced by the latter. The two factors worked together, competed and compromised, and contributed to the present Renfu Dialect.