Lenition is a weakening process which causes consonants to become more sonorous with wider constrictions. Lenition mostly occurs in phonetically short positions. In Beijing Mandarin, the three retroflex obstruents /ʂ ʈʂʰ ʈʂ/ optionally lenite to a more sonorous approximant [ɻ] in onsets. It is argued that lenition occurs in unstressed positions, where syllables surface as phonetically short. Furthermore, prosodic structures in Beijing Mandarin are conditioned by, but not perfectly mapped from, morphosyntactic structures, resulting in a morphosyntax-prosody mismatch. Morphological compounds and syntactic phrases with the same hierarchical structures have identical prosodic structures. Lenition occurs on the weak syllable of a trochee, and an analysis in Harmonic Grammar is derived to account for a gang effect problematic for classical Optimality Theory with strict domination. Foot recursion is also necessary to derive the prosodic structures that capture the observed lenition sites. As a further theoretical implication, lenition provides insight on word-level prosody in tonal languages, where stress is often not distinctive and demonstrates that syllables with the same tones can contrast in strength.