Microstructure and mechanical properties including grain size of initial austenite, size of martensite slat block and proportion of small angle and large angle grain boundaries for C250 steel with  different forging ratios were studied by means of metallographic microscopy, EBSD technique and mechanical proparty testing. The results show that the sizes of grain and martensite slat block of C250 steel are refined by forging process effectively. The grain size is refined to level 12 for C250 steel with no deformation and grain size of level 0 under the same process by large forging ratio deformation. At the same time, the martensite slat blocks of specimens with medium and large forging ratios are refined significantly, the average grain size of martensite slat block is refined to about 7 μm. The proportion of small angle grain boundary which is less than 10° decreases, the proportion of large angle grain boundary which is between 50° to 60° increases gradually, and the small angle grain boundary changes into large angle grain boundary gradually with the increasing of forging ratio. The impact toughness of C250 steel increases with the decreasing of grain sizes.