We introduce the Composition of LLPS proteome Assembly by Proximity labeling assisted Mass spectrometry (CLAPM) to address such unmet need in this filed. We demonstrate CLAPM for instantaneously labeling and monitoring in situ proteome shifts within intracellular droplets that undergo spatiotemporal LLPS. CLAPM offers a versatile method to decipher proteins involved in LLPS assembly and enables the accurate characterization of dynamic proteome in living cells.