This paper analyses the expressive affective prosody of children suffering from Mental Retardation (MR) in comparison to typically developing children in spontaneous speech, with the hopes of furthering research into the following: (1) whether or not there is damage to the affective prosody of MR; (2) whether the development of the affective prosody of MR develops with vocabulary, syntax and other linguistic factors. For the experiment, 18 MR were chosen, 18 typically developing children matched to the MR group for chronological age (CA), 18 typically developing children matched to the MR group for language abilities (LA) were recruited. Subjects are required to say a narrative using the picture book Frog, Where are you (Mayer, 1969). Data were collected from each child individually in a quiet room, using the voice recorder recorded the spontaneous speech of the children in the entire narrative task, intercepted the climax of the narrative as the experiment material. Using the cross-platform multifunction phonetic software PRAAT, the pitch range and vowel duration of the climax section were extracted as two major influences in affective prosody, the data from which was used to make a database in Microsoft Excel, then spss15.0 was used for statistical analysis, undertaking a one way ANOVA. Results showed: the pitch range of MR showed a significant difference with the two typically-developed groups of p=.000<.001, significantly higher than the two typical groups; with regard to the vowel duration, the MR showed a significant difference with CA group of p=.000<.001, but the difference with LA group was not significant (p=.371>.05), both the MR and the LA group were found to use significantly longer vowels than the CA group. According to the experimental data, the following conclusions were made in spontaneous speech : (1) The affective prosody of MR shows damage in spontaneous speech, displaying atypical characteristics, the difference in pitch range being the most atypical display. (2) The affective prosody of MR develops significantly later than vocabulary and syntax, with a concentrated performance in the pitch range, indicating that affective prosody possesses modularity in spontaneous speech. This paper conducted research into the affective prosody of MR in spontaneous speech from the perspective of neurolinguistics, which is the first time in the country. In addition, this paper used the method of experimental phonetics, using of phonetic software PRAAT to calculate the acoustic parameters, and giving the qualitative and quantitative analysis. This kind of research into the phonetic problems of MR has not yet been otherwise researched domestically.