The invention relates to a nano suspension for oral protein immunization and a preparation method thereof, and belongs to the field of medicine. Bovine serum albumin (BSA) is taken as the model drug, PLGA is taken as the carrier, and a solvent volatilization method is adopted to prepare a BSA loaded MCS/PLGA/BSA nano suspension. The particle size of the suspension is 532.8 nm, the zeta potential is 28.92 mv, and the encapsulation rate is 89.34%. Mannosylated chitosan (MCS) is adsorbed on the surface of PLGA/BSA nano particles through a static effect, thus the stability of nano particles in gastrointestinal liquid becomes more stable, moreover, the stay time in intestinal cavity is prolonged, and the nano particles with positive charges can be absorbed by cells more easily. Nano particles are transferred by epithelium related with follicle to PP nodes; through the combination between mannose ligands and mannose acceptors on antigen-presenting cells (APCs), the intake of APCs is increased, and thus the immunization reactions are induced.