The invention relates to an antimicrobial-free selenium-enrichment piglet mixed feed additive preparation method. A formula of the antimicrobial-free selenium-enrichment piglet mixed feed additive comprises the ingredients by weight percent: 1.8 to 2.8% of L-tyrosine, 2.3 to 3.3% of natural vitamin E, 1.2 to 2.2% of rhizoma pinelliae, 2.3 to 3.3% of lactobacillus buchneri, 1.5 to 2.5% of trisodiummonohydrogen, 2.5 to 3.5% of saw palmetto extractive, 2 to 2.8% of bacillus laterosporus, 1.3 to 2.3% of mile swertia herb, 2.6 to 3.6% of radix berberidis, 1 to 2% of enterobacter cloacae, 0.5 to 1.5% of calcium propionate, 1.2 to 2.2% of radix angelicae sinensis, 0.3 to 1.3% of dimethyl ancymidol menadione sulfite, 0.8 to 1.8% of bacillus lentus, 0.9 to 1.7% of manganese carbonate, 0.6 to 1.6%of snake gall extractive, 0.9 to 1.8% of chromium yeast, 1.2 to 2.2% of rhynchophylline, 0.9 to 1.9% of probiotic, 0.6 to 1.6% of selenium yeast and 73.6 to 54.1% of water. The antimicrobial-free selenium-enrichment piglet mixed feed additive disclosed by the invention has better functions of promoting mitosis and differentiation proliferation of lymphocyte and macrophage, increasing cell factorsand maintaining intestinal mucosa completeness and mammary epithelial cells.