To achieve a better understanding of the relationship between syntactic parsing and prosodic phrasing in speech production cross-linguistically, we investigated how syntactic constituents map onto a high-dimensional acoustic space of prosodic phrasing in two read-speech corpora of Mandarin and English with syntactic annotations. The left and right edges of the constituents from the syntactic parsings were used as a proxy for the relative strength of the syntactic boundaries. A wide range of acoustic cues capturing pauses, duration cues, F0, energy, and voice quality cues were extracted. Our results showed that there is a clear correlation between the strength of syntactic boundary and prosodic phrasing, and the syntax-prosody mapping is much stronger for the right boundaries than for the left boundaries. Moreover, the prosodic realization of syntactic boundaries is gradient (especially for right boundaries), and acoustic cues scale up or down collectively to indicate different extents of phrasing, rather than being specific to certain levels of phrasing. We discuss the findings' implications in relation to the prosodic hierarchy and the nature of the prosody-syntax interface.