The invention relates to a preparation method of polysiloxane modified polyurethane water dispersion. According to the preparation method, IPDI (isophorone diisocyanate) is melted, then, polyether 210 and polyether 220 are added, and the mixture reacts for 25-30 min at the temperature of 70 DEG C-80 DEG C; a polysiloxane modifying agent is added, and the mixture reacts for 30 min; a dimethylol propionic acid solution is added, and the mixture reacts for 25-30 min at the temperature of 70 DEG C-80 DEG C; ethylene glycol is added, the mixture reacts for 40-60 min, and a modified polyurethane prepolymer is obtained; and the prepolymer is cooled to the room temperature, acetone is added, the mixture is uniformly stirred at the normal temperature, then, triethylamine is added, an acetone solution of the prepolymer is obtained and added into a water solution of diethylenetriamine after 15-30 minutes, the mixture is stirred and subjected to emulsification chain extension, acetone is removed, and the polysiloxane modified polyurethane water dispersion is obtained. The prepared polysiloxane modified polyurethane water dispersion has excellent water resistance, ageing resistance, weather resistance, high-low temperature resistance and chemical solvent resistance.