A kind of innovative Airbag, Roof Airbag (RAB), with external straps is studied via virtual engineering, which provide protection to rear-seat occupants in frontal impact. Frontal sled model with AF05 female dummy is generated in LS-DYNA, which is well correlated with full vehicle frontal crash tests in terms of the kinetics and injuries of the dummy. In addition, it is adapted for AF50 and AF95. Based on above models, different configurations of restraint system are studied, different levels of load limiter, belt with or w/o pretensioner, with or w/o RAB for instance. It can be summarized that roof airbag with low level of belt load limiter and pretensioner could provide protection to most size of rear-seat adult occupants as good as that of front-seat occupants in frontal impact.