The invention discloses a martensitic heat-resistant cast steel material which contains B and N and is used for an ultra-supercritical steam turbine and a preparation method of the martensitic heat-resistant cast steel material, and relates to a heat-resistant cast steel material and a preparation method thereof. The heat-resistant cast steel material comprises, by weight, 8.5-9.5% of Cr, 2.50-3.50% of W and 2.80-3.30% of Co and does not comprise RE. The preparation method comprises the steps that raw materials are weighed and subjected to primary refining at first, then first-stage refining and second-stage ladle refining are conducted, and heat treatment and performance heat treatment are continued after casting. The high temperature strength of the material is improved by the Cr elements, and delta-ferrite phases are not generated; arc furnace primary refining and two-step refining are beneficial to composition adjustment and optimization, and inclusions are removed; and through annealing and performance heat treatment, an original casting structure can be improved, harmful delta-ferrite phases are prevented from being generated, the performance of the material is improved. Themethod is applicable to preparation of the martensitic heat-resistant cast steel material which contains B and N and is used for the ultra-supercritical steam turbine.