Liquid-liquid phase separation of biomolecules can be induced in vitro by selecting specific concentrations and environmental conditions. Phase diagrams relating the temperature, pressure, etc. to the concentrations of proteins and co-solutes aid in identifying the specific phase separation conditions for target proteins. However, it remains challenging to determining complete phase diagrams for protein liquid-liquid phase separation. Here, we combine optical microscopy and in situ Raman spectroscopy to quantify the temperature-dependent concentrations of the light and dense phases of human γD-crystallin and construct a complete phase diagram. The method can be extended to systems in the presence of co-solutes, which enables the quantitative description of the partitioning of a co-solute in the light and dense phase of proteins.