Objective The upper arms are main defensive parts in knife slash injuries. This work was to explore the effluence of knife sharpness on forearm wounds in slash injuries in quantity. Methods Finite element models of an upper arm and knifes with 3 sharpness (A, B, and C, from the sharpest to bluntest) were developed based on human CT images and a prototype of a slash knife. Computer simulations were performed to analyze contact forces, wound dimensions and energy changes during slashing with velocity of 4 m/s and 0.01 s duration on the forearm using the 3 kinds of knife. Results The blades reached the ulna at about 65 ms, 85 ms, and 95 ms for A, B and C respectively. Moreover this, the corresponding slash forces were 846 N, 1064 N, and 1865 N; the wound lengths were 135.64 mm, 105.47 mm and 99.23 mm; and the wound depths were 38.77 mm, 27.81 mm, and 18.74 mm. Compared to B or C, the total energy and inner energy of the model system for A were obviously greater, but kinetic energy for A was less. Conclusions A novel method based on finite element analysis has been developed for quantitatively assessing wounds formation in knife slash upon upper arms. The wound formation is slowed, depth decreased and slash force increased while the knife is blunted.