A series of studies have advertized the benefit of word segmentation in Chinese sentences comprehension for children with reading difficulty. This study attempts to identify the major cognitive factor contributing to this developmental difference among the children with the same age. A sentence reading with word judgment task was designed for a group of children with reading difficulty and a control group of children with normal reading capacity. In each trial of word judgment, a stimuli sentence could follow a word appearing in the critical region, a word appearing in the critical region but without meaning, a word appearing in the uncritical region, or a word disappearing in the sentence. For the words in the critical region, there is a ambiguous segmentation between them, and the children takes more cognitive effort to deny the word without meaning. The current results show the only difference between the two groups on the words without meaning. After excluded the factors such as intelligence and vocabulary, the inefficiency of attention would be the critical to reading difficulty. An improvement method based on a cognitive processing of Chinese sentence reading is suggested in this presentation.