The invention discloses a method for preparing high-pressure-resistant and high-slump-loss-resistance underwater C35-C40 concrete poured under an an ultra-deep water environment. The method comprisesthe step of mixing cement, coal ash, slag micropowder, natural medium sand, stones, additives and water, wherein the dosage proportion of the components are as follows: 230 to 270 parts of cement, 65to 85 parts of coal ash, 95 to 125 parts of a slag micropowder, 780 to 810 parts of natural medium sand, 960 to 1010 parts of stones, 5.74 to 6.15 parts of additives and 160 to 165 parts of water; theunit of each component is kg/m<3>. Through special blending of the additives, the preparation of a cementing system and the adjustment of aggregate gradation and sand rate, the setting time of the high-pressure-resistant and high-slump-loss-resistance underwater C35-C40 concrete is prolonged by more than 3 hours compared with that of the conventional concrete, and the high-pressure-resistant andhigh-slump-loss-resistance underwater C35-C40 concrete still can maintain high flowability and constructability under the underground ultra-deep environment (80 to 150 m), avoids segregation and weeping phenomenon and has high slump loss resistance.