The invention discloses an expandable type building fireproof coating and a preparation method thereof. The preparation method comprises the following steps: weighting unsaturated polyester resin andthen adding into a stirring reaction kettle; adding styrene, methyl ethyl ketone peroxide, cobalt naphthenate and liquid paraffin into the reaction kettle in turn while stirring, and then reacting while stirring for 20-40min; weighting expandable graphite, ammonium polyphosphate, pentaerythritol and melamine and then adding into a grinding mill for grinding; weighting filler and then adding into the grinding mill for grinding; pouring the filler and the mixture of ammonium polyphosphate, pentaerythritol and melamine into the stirring reaction kettle, and quickly stirring and mixing for 30-60min; adding auxiliaries and solvent, and then stirring and mixing again. According to the invention, different modified resins are stirred and mixed at different ratios, so that the fireproof time of the coating is prolonged, and meanwhile, the fireproof coating has anti-ultraviolet, antibacterial, sterilizing and anticorrosion effects, and the service life of the coating is prolonged.