The present invention relates to a method for preparing a high strength thick steel plate for a large oil storage tank in a low cost, the method particularly applies to a high strength thick steel plate for a large oil storage tank, which is welded by line energy which is equal or less than 120 KJ/cm. The weight percentages of chemical constituents are that carbon is 0.01-0.12 percent, silicon is 0.15-0.35 percent, manganese is 1.45-1.80 percent, phosphorus is equal or less than 0.010 percent, sulfur is equal or less than 0.003 percent, aluminum is 0.02-0.06 percent, vanadium is 0.02-0.06 percent, nickel is 0.20-0.40 percent, nitrogen is 0.003-0.006 percent, and the other is ferric, other alloying elements and inevitable impurities. The present invention uses on-line quenching and off-line tempering processes, thereby reducing valuable alloying elements, obtaining high strength high toughness thick steel plate which has excellent welding performance and does not need to preheat above 0 DEG C, reducing the producing cost, saving energy, increasing production efficiency and improving welding construction conditions, and the invention is capable of being widely used for constructions of a 10-30 cubic myriametre of large oil storage tank.