During an in-vessel loss of coolant accident (LOCA) of helium-cooled blanket in fusion reactor, the high pressure helium gas ingress into the vacuum vessel (VV) will produce highly underexpanded jet, resulting in the resuspension and migration of the radioactive dust deposited in the VV. In this study, the flow field in VV and the influence of friction velocity were simulated and analyzed by using ANSYS Fluent code. In addition, the differences between in-vessel LOCA and loss of vacuum accident (LOVA) were compared. The results showed that the pressure of the VV would reach the pressure relief limit of vacuum vessel pressure suppression system (VVPSS) rapidly after LOCA. With the development of LOCA, the Mach disk continued to move towards the equatorial break of the VV, and its shape continued to shrink, with the number of structures gradually increased. Compared with LOVA, the LOCA had a higher friction velocity, resulting in a greater probability of dust resuspension and radioactive risk.