Triblock Janus particles are prepared by two-step seeded emulsion polymerization against eccentric polymer hollow particles. Two silica bulges are sequentially grown at the eccentric zone transiting from the thick shell to the thin shell. The angle between the two silica bulge axes is tunable within 0-180 degrees, determined by the eccentricity degree of the PS/PDVB hollow seed particles. The two silica bulges and the PS/PDVB domain of the triblock Janus particles can be selectively modified to enrich composition and functionality. As an example, the paramagnetic PS/PEO modified triblock Janus composite particle can serve as a functional solid surfactant to manipulate emulsion droplets with a magnet and deliver desired materials toward the interface.