Expression type for mixed-use of Dongba characters can be divided into dominant type and parallel type. Causes for mixed-use includes three points: 1. similar or compatible patterns are the precondition for mixed-use; 2. certain relation exists among semantics of words represented by characters, or characters belong to the same classification in terms of meaning or have correlative meanings; 3. outcome is obtained during self-adjustment transitional stage of internal character of writing system. There are three solutions to solve the problem of similar characters: 1. re-distribution of characters’ functions; 2. integration of characters; 3. reconstruction of expression which is embodied in two aspects, including ideographical and phonetic expression transited from ideographical expression to additional phonetic complement, and transition from ideographical expression to pure phonetic expression.

合文是自源的表意文字發展不成熟階段的一種特殊的文字現象，東巴文也不例外。東巴經中的合文現象尤其普遍，並且東巴經較爲完整地保留了合文形成發展的歷時軌跡。究其形成原因，可以從內因和外因兩方面去探討。其內因表現在：文字發展不成熟，文字單位與語言單位存在參差關係；概念的整體性導致文字符號書寫時緊密連接；遵從書寫時的“事理性”導致合文。其外因主要表現在：書寫空間的局限造成字符的臨時合書；不同符號之間長期並聯使用易導致合書；因具有相同部件而合書；迎合東巴經整體書寫形象性、趣味性的特點。