The invention relates to an alcohol-free beer and a preparation method thereof, and aims to solve the technical problem that the alcohol-free beer in the prior art cannot easily achieve an alcohol content of 0.00 percent and has a long production cycle, a high production cost and the like. The beer of the invention consists of pre-treated beer solution and carbon dioxide filled in the solution, wherein the pre-treated beer solution consists of the following raw materials in percentage by weight: 15 to 25 percent of 10 to 13 percent wort, 0.5 to 1.5 percent of dietary fiber, 2 to 3 percent of syrup, 0.01 to 0.05 percent of acidulant, 0.01 to 0.02 percent of natural flavor, 0.04 to 0.06 percent of pigment and the balance of water; and the pre-treated beer solution also comprises 0.35 to 0.65 percent of carbon dioxide. The alcohol-free beer of the invention has the advantages of mild hop and malt aroma, clean and refreshing mouthfeel, no peculiar smell and capability of greatly reducing the production cycle and reducing cost.