The invention discloses an extremely-thick low alloy steel plate for an E-level high purity ocean platform and further relates to a production method of the steel plate. The steel plate comprises, by weight, 0.10%-0.18% of C, 0.15%-0.35% of Si, 0.90%-1.60% of Mn, 0-0.02% of P, 0-0.005% of S, 0.020%-0.030% of Nb, 0.010%-0.080% of Al, 0.035%-0.065% of V, 0-0.025% of Ti, and the balance Fe and inevitable impurities. The production method of the steel plate includes the following steps of electric furnace smelting, LF / VD refining, continuous casting, electroslag remelting, steel ingot heating, rolling, water cooling after rolling, heat stacking, heat treatment and finished product obtaining. The maximum thickness of the steel plate obtained through the method can reach 350 mm, and the characteristics of being good in welding property, excellent in low temperature impact toughness and good in thickness directional property are achieved. Meanwhile, steel is clean, performance evenness is good, high flaw detection and performance requirements can be met, and a wide application prospect is achieved.