Abstract:The "Zhangbaba" custom inherited in the wood construction technique of Miao and Tujia nationalities in Sichuan-Chongqing area, is a type of construction custom which uses "eight" as the last number in the size design of traditional buildings. This construction custom controls the architectural space scale with concise and easy-to-remember size numbers, reflecting the experience accumulation and wisdom summary of construction techniques of local traditional buildings. It’s also a continuation and important presentation of the modular system of traditional buildings. This article takes the traditional dwellings of Mugedao Village in Xuyong County and Dazhai Village in Xiushan County as examples. Through the detailed field investigation and carpenter interview, it explores the specific representation mechanism of the "Zhangbaba" custom in traditional wood construction technique, then analyses the relevance and difference of the "Zhangbaba" custom in different regions, trying to clarify some important rules and characteristics of the traditional building construction of the Miao and Tujia nationalities in Sichuan-Chongqing area.