With taking the technology measures such as increasing Mn content in steel from 1. 55% ~ 1.65% to 1,75% 〜1. 85% , replacing 0. 20% ~0. 25% Ni in steel by 0. 25% 〜0. 35% Cr, adding 0. 01 % ~0. 04% Ti for microalloying, controlling [ N] ≤80 x 10-6and [O] ≤15 x 10-6by RH vacuum treatment, decreasing casting secondary cooling water rate from 0. 11 L/kg to 0. 08 L/kg and modifying water distribution ratio to increase temperature of bloom at outlet from 620 ~ 680 ℃ to 700 ~ 750 °C , bloom cooling with shielding and bar products slow cooling, the production cost of R3 grade mooring chain steel is reduced, the longitudinal cracks of 350 mm x 470 mm cast bloom are eliminated and the strength and the toughness at -20 ℃ of products all meet the requirement of standard.