The emulsion droplet solvent evaporation method has been used to prepared the PSt/PGMA monodisperse janus particles, as solvent DCM evaporated, the spherical droplets phase separated, resulting in janus morphology composite particles. The different janus morphology PSt/PGMA composite particles were prepared by controlling the molecular weights of PSt and PGMA. The effect of different type of the emulsifier and its concentration on the particles morphology were researched. Fluorescence microscopy observation and EDS energy dispersive spectrometer results confirmed that the large hemisphere mainly is PGMA, the small hemisphere mainly is PSt, the phase-separation process in these two polymer is not yet complete, with the molecular weight increased, PGMA tend to engulf PSt component.