The invention discloses an S275NL high-tenacity and fine-grain structure steel plate and a preparation method thereof. The steel plate comprises, by weight, 0.07%-0.15% of C, 0.20%-0.40% of Si, 0.75%-1.00% of Mn, 0%-0.025% of P, 0%-0.020% of S, 0.05%-0.10% of Nb/Ti, 0%-0.050% of Als, and the balance Fe and inevitable impurities. The preparation method of the steel plate includes the following steps of top-quality molten iron smelting, molten iron pretreatment with a Kanbara reactor (KR), smelting with a converter, refining with a ladle furnace (LF), vacuum degassing, continuous casting, heating, rolling, rapid cooling under accelerated cooling control (ACC) and normalizing heat treatment. According to the high-tenacity and fine-grain structure steel plate, the contents and the ratios of alloy elements of C and Mn are adjusted, Nb/Ti alloy elements are added appropriately, the P content and the S content are controlled strictly, and the measures are matched with the rolling and normalizing processes, so that precious alloy elements such as Ni and Mo are prevented from being added to the steel plate, and the requirements of a low carbon equivalent, high tenacity, good welding performance and the like of the steel plate can be met; the production processes are simplified, the production cost is reduced, and the method is suitable for large-scale production operation.