[en] For the safety issue for top-up injection, the trajectory of the injection beam was tracked to find out in what kinds of conditions the injection beam would get through the synchrotron light beam pipe and cause radiation hazards. The forward tracking method is used with Accelerator Toolbox (AT) code. We enlarge injection beam phase space volume and reduce the beam line element to simplify the calculation while covering all the possibility. According to this simulation, the interlock criteria are set up for top-up operation. (authors)