A random generation algorithm was used to steel fiber， and then the meso-scale finite element numerical mode of three point bending beam of random and aligned steel fiber reinforced cementitious composites （SFRC， ASFRC） were established. The whole process of loading and fracture of SFRC specimens and ASFRC specimens with different fiber contents were calculated. The fiber stress at the cracked section of the three point bending beam was analyzed， and the meso-scale reinforcement mechanism of aligned steel fiber was studied. The results show that the whole load-crack opening displacement curves of SFRC specimens and ASFRC specimens obtained from test and numerical simulation are in good agreement and the error of the peak load is within 10%. The peak load and the maximum fiber force of the two kinds of specimens both increase with the increase of fiber content. When the fiber content is 0.8%， 1.2% and 2.0%， the peak load of ASFRC specimens increased by 75%， 111% and 141% compared to SFRC specimens， and the maximum fiber force of ASFRC specimens increased by 202%， 144%， and 127% compared with SFRC specimens. Aligned steel fiber can effectively improve the fracture performance of cementitious composites， significantly increase the fiber efficiency， and delay the propagation of cracks.