The invention discloses a composite foamed cement heat retaining panel using high-volume fly ash and a preparation method thereof; the composite foamed cement heat retaining panel is prepared by 60-80 parts of cement, 20-40 parts of fly ash, 0.6-3.0 parts of moisture repellent, 0.12-0.30 part of a water reducing agent, 0.8-3.0 parts of an exciting agent, 0.3-0.50 part of anti-crack fiber, 30-60 parts of water and 4-8 parts of a foaming agent (by weight); the preparation method comprises the following steps: weighing each raw material; adding the water, the anti-crack fiber, the moisture repellent, the cement, the fly ash, the water reducing agent; stirring a mixture; adding the exciting agent; stirring the mixture; adding the foaming agent; stirring the mixture so as to prepare mixed slurry; then, injecting the mixed slurry into a mould; putting the mould statically for 4-5 hours; removing the mould; after 24 hours, cutting and packaging a product; and maintaining the product until age is 28 days. According to the invention, a part of the cement is substituted by fly ash; cost is low; environment pollution is reduced; and social efficiency and economic benefit are obvious.