The invention relates to a preparation method of an LED backlight source heat dissipation substrate material and belongs to the heat dissipation substrate material preparation technical field. According to the preparation method, aluminum nitride ceramic powder, aluminum powder and diatomite are selected as main raw materials and are pressed into a blank; calcination and annealing are performed on the blank, so that an aluminum nitride ceramic composite substrate is obtained; alkaline washing, acid immersion and sensitization treatment are performed on the aluminum nitride ceramic composite substrate, so that a sensitized aluminum nitride ceramic composite substrate can be obtained; thermal conductive polymer made of epoxy resin is applied onto the sensitized aluminum nitride ceramic composite substrate; and after curing is carried out, the LED backlight source heat dissipation substrate material can be obtained. The heat conductivity coefficient of the LED backlight source heat dissipation substrate material prepared by using the preparation method is high and can be higher than 62W/m.K. The LED backlight source heat dissipation substrate material has excellent heat dissipation performance and good insulation performance; and the surface resistance value of the LED backlight source heat dissipation substrate material is higher than 15 megohms.