The invention relates to a diatomite building thermal insulation material prepared from the components in parts by weight: modified diatomite, coal ash, quartz sand, anti-cracking fibers, plant cellulose, a flame retardant, a foaming agent and a binding agent. The diatomite building thermal insulation material is prepared by stirring and uniformly mixing the modified diatomite, the coal ash, the quartz sand and the anti-cracking fibers according to the weight ratio; then, adding water, carrying out stirring while heating, and slowly adding the plant cellulose, the flame retardant, the foamingagent and the binding agent; and then, carrying out hot press molding to obtain the diatomite building thermal insulation material. The modified diatomite is used as a main raw material, so that the diatomite building thermal insulation material is high in porosity, good in thermal storage property and stable in chemical property; due to the addition of the anti-cracking fibers and the plant cellulose, the mechanical strength and flexibility are improved; due to the use of the binding agent and the flame retardant, the fire resistance of the thermal insulation material is improved; and the coal ash and waste paper powder are added as auxiliary materials, so that the utilization ratios of waste paper and coal ash are increased, and the diatomite building thermal insulation material is energy-saving and environment-friendly. The diatomite building thermal insulation material is simple in preparation method, low in cost, light in weight and convenient to construct and is an environment-friendly material.