The invention provides a preparation method for a nanometer zinc oxide modified cotton fabric. The preparation method is characterized by comprising the following steps: after performing swelling treatment on the cotton fabric, performing flame-proof treatment on the cotton fabric, wherein the flame-proof treatment comprises the step of preparing at least two solutions in zinc ion concentration of 10-20wt%, wherein the zinc ion concentrations of the solutions are unequal to each other; soaking the fabric into the solutions in sequence of gradual increasing of zinc ion concentrations of the solutions; taking out and then suspending in a sealed container filled with ammonium hydroxide for 5-15min; taking the cotton fabric out and naturally placing till ammonia gas volatilizes; baking; cleaning and drying, thereby obtaining the nanometer zinc oxide modified cotton fabric. The raw materials selected according to the invention are non-toxic or low-poison; the generated nanometer zinc oxide is non-toxic to human body; the operation is simple; the raw material source is wide; the cost is low; the cotton fabric after being treated has excellent flame retardant property and contains no formaldehyde; the preparation method is an environment-friendly flame-proof treatment method.