This paper investigates the comprehension of long and short passives in 15 Mandarin preschool children with Developmental Language Disorder (DLD) (aged 4;2–5;11 years), 15 Typically Developing Age-matched (TDA) (aged 4;3–5;8 years) children, and 15 Typically Developing Younger (TDY) (aged 3;2–4;3 years) children by using the picture-sentence matching task. The results reveal that children with DLD encounter more difficulty comprehending long passives compared with short passive, that they perform worse on the comprehension task than TDA children and TDY children, and that this population is more likely to commit thematic role reversal errors and point to pictures with the incorrect agent (patient) than typically developing children. Given that Mandarin passives are Topic Structures, we maintain that children with DLD are insensitive to the edge feature of the moved element in long passives, leading to Relativized Minimality effect and causing the asymmetry between the comprehension of long and short passives. These results align well with the Edge Feature Underspecification Hypothesis. Errors found in the children with DLD in the comprehension task point toward impaired syntactic knowledge and the lexical semantic deficit.