The invention claims a flame-retardant and corrosion-resistant fiber bamboo substrate and a preparation method thereof. The preparation method comprises the following steps of: 1) cutting raw bamboo into bamboo filaments; 2) flame-retardant treatment: soaking the bamboo filaments prepared in Step 1) in aqueous solution of a flame retardant; 3) drying: drying the flame-retardant treated bamboo filaments at 55° C. to 65° C. until the absolute water content is not more than 12%; 4) carbonized pyrolysis: feeding the dried bamboo filaments into a carbonized pyrolysis kiln, to be high-temperature treated according to a pyrolysis gradient; and, 5) sequentially gumming, post-gumming drying, pressing, curing, maintaining and splitting to obtain a bamboo substrate. The bamboo substrate has high stability, no cracks on the product surface, enhanced corrosion resistance and excellent flame retardance, and may be used in various climate conditions and environments.