Spatial ability has been shown the positive correlations with the mathematics achievement of typically developing children. However, whether there is a relationship between spatial ability and mathematics achievement in deaf children and the mechanisms that underlie the relationship remain unknown. 256 deaf children in Grades 3 to 9 in two special education schools in China performed a series of cognitive and mathematical tests. After controlling for age and gender, we found that spatial ability, processing speed (both accuracy and reaction time), and intelligence were significantly related to mathematics achievement. Moreover, processing speed and intelligence were found to mediate the relationship between spatial ability and mathematics achievement in deaf children. The mediating effect of intelligence was significantly greater than the serial mediating effect of processing speed (accuracy) and then intelligence; both the mediating effect of processing speed (reaction time) and the mediating effect of intelligence were significantly greater than the serial mediating effect of processing speed (reaction time) and then intelligence. These findings suggest a process through which processing speed can decrease intelligence and identify the mediating effects of processing speed (both accuracy and reaction time) and intelligence in the relationship between spatial ability and mathematics achievement in deaf children.