The invention provides Cr5 series hot work die steel which comprises, by weight, 0.40-0.48% of C, 0.20-1.2% of Si, 0.40-0.80% of Mn, no more than 0.020% of P, no more than 0.010% of S, 4.80-5.50% of Cr, 1.40-2.0% of Ni, 0.80-1.50% of Mo, 0.50-0.70% of V, 0.50-1.00% of W, 0-0.75% of Co, 0.01-0.20% of Al, 0.02-0.20% of Nb, and the balance Fe. The chemical composition formula of the novel Cr5 series hot work die steel provided by the invention is recorded as the YB5 series, and compared with existing Cr5 series (H13) hot work die steel, the YB5 series die steel is higher in C content and Mn content and lower in Si content (YB5A and YB5C), Mo content and V content, and Ni, W, Co (YB5C) and Al (YB5A and YB5C) are added. Accordingly, the toughness of the steel is improved, and the high-temperature strength and creep resistance are improved.