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Atomistic cracks have been the subject of material intensive research in recent years due to the fast development of nanomaterials. In order to have a better understanding of the fracture mechanisms of body-centered-cubic (BCC) metal, the multiscale quasi-continuum method (QC) is employed to analyze the nano-sized crack of BCC tungsten. The mode Ⅱ crack of Tungsten(W) in {110} planes along the [111] direction is simulated. The load-displacement curve and atom displacement images for each loading step are presented. The generation of partial dislocations, the nucleation and emission of perfect dislocations and the movement of dislocations in crack tip have been observed. Simulation results show that partial dislocations will produce before perfect dislocation nucleation; each drop point of the load-displacement curve corresponds to the nucleation and emission of a perfect dislocation; dislocation nucleation happens several times along with the dislocation launching; the increasing number and rapid movement of dislocations eventually lead to mode Ⅱ fracture. According to the simulation results, the curve of dislocation position vs. displacement is presented, and the movement characteristics of dislocations are analyzed. The results show that all the dislocations will launch after a new dislocation nucleation, indicating that a new dislocation nucleation will promote dislocation movement, and dislocation movement will speed up with the increase in the number of dislocations. In addition, the phenomenon and mechanism of dislocation in BCC metal is analyzed according to the theory of crystallology and Rice’ theory of unstable stacking fault energy. Finally, the forces on and between dislocation are discussed. By calculating the force balance equation in microscale, the initial equilibrium position of the dislocation is forecasted, and the movement mechanism of dislocations near the crack tip is explained, which coincides well with the simulation results.