The study was supposed to discover the cognitive processing mechanism of MLD. Math learning disability (MLD) is an important area of learning disability, accounting for large percentage of students in the middle school. Now there emerges a leading paradigm for carrying out research on MLD from the viewpoint of information processing. The study was designed to find out whether the MLD students have deficit on short-term memory span or the working memory span. The criteria for defining the MLD or math learning disorder of this study is: the math score was lower than 20% of the total score, with normal IQ, and without the obvious organic deficit, emotional disorder and learning motivation disorder. There are 103 students participating the study, with 49 MLD students and 54 students who are good at math (with average age is 12.57-year-old). The study designed the three experiments such as the word series task to measure short-term memory, reading span task to measure phonological working memory, and backward digit span task to measure digit working memory. The reading span task was designed on the computer by E-prime software, while the other two tasks were oral tests. Each participant spent 30 minutes finishing the experiments with the guide of psychological graduate students. The results of the study showed: (1) there was no significant difference between two groups in word series task (t＝-1.59, p>.05), but significant difference between two groups in reading span tasks (t＝-2.38, p<.05) as well as backward digit span task (t＝-4.69, p<.01 ). (2) Correlation analysis showed reading span task and backward digit span task had high correlations (r=.24, p<.05), and the word series task had low correlation with the reading span task (r=.19, p>.05) and backward digit span task (r=.05, p>.05). (3) The regression analysis indicated that the backward digit span task which measured the digit working memory can better predict math achievement than the other two tasks which measured the short-term memory and reading working memory. (4) The developmental trend of memory span among the junior school students was no significant. The conclusion of the study was the MLD students couldn’t finish the tasks with high loaded working memory demand, but they were capable to finish some simple cognitive tasks such as short-term memory task, which meant that the counter interference and attention control abilities deficit was the core deficit in the MLD.