The invention discloses an ultra-thick electroslag plate applied to ammonia synthesis tower equipment and a preparation method thereof. The ultra-thick electroslag plate is prepared from the followingchemical components in percentage by mass: 0.10-0.15% of C, 0.50-0.58% of Si, 0.40-0.50% of Mn, less than or equal to 0.006% of P, less than or equal to 0.006% of S, 1.40-1.50% of Cr, 0.60-0.65% of Mo, 0.15-0.20% of Ni, less than or equal to 0.20% of Cu, less than or equal to 0.005% of As, less than or equal to 0.004% of Sn, less than or equal to 0.003% of Sb and the balance of Fe and unavoidableimpurities. According to the electroslag plate, the contents of C, Si and Mn in steel are reduced; the contents of Cr and Mo are increased; through adoption of multi-component composite microalloyingwith low contents of C, Si and Mn, the effects of alloy elements Cr and Mo are enhanced; the strength of the steel plate is ensured; meanwhile, the low-temperature impact toughness of the steel plateis improved; the content of C plays a main role in mechanical performance of the steel; C and other alloy elements can form carbide in the steel, so that the strength of the steel plate is improved;the contents of harmful elements P and S and residual elements such as As, Sn and Sb and the like are reduced as far as possible under the condition that the equipment and the process are met, so thatthe steel is pure and is excellent in mechanical property.