Abstract: Palladium-poly (methyl methacrylate-2-acrylamido-2-methylpropanesulfonic acid) (Pd-P(MMA-AMPS)) composite microspheres，containing amide group and sulfonic acid group，were prepared in the palladium (Ⅱ) chloride (PdCl₂) solution without initiator，emulsifier and reductant by ultrasonic irradiation emulsifier-free emulsion polymerization. The effect of the mass ratio of monomers on the morphology of the composite microspheres was investigated，and the samples were characterized by XRD，TGA，FTIR，TEM and Zeta-sizer. The reaction mechanism of preparation in situ was also proposed. The results show that Pd²⁺ can be reduced to Pd⁰ particles and the monomers are polymerized simultaneously. There is interaction between carbonyl group in the composite copolymer and metallic palladium. The composite microspheres with diameter of 0.76μm and 20% of palladium mass fraction are successfully synthesized under the optimized experimental condition. The mono-disperse metallic palladium particles are coated by the polymer matrix effectively.