Based on the NS traffic model, we propose a cellular automaton traffic model for four-aspect fixed autoblock systems. We simulate the traffic phenomenon of train delay. We also investigate the effects of main factors including train interval, proportion of freight train, dwell time of the train in the station and unmber of platforms. Simulation results show that: (i) the freight train can restrain the running velocity of the passenger train following it; (ii) the delay may be relaxed by adjustment of the train's dwell time in the station; (iii) the increase in number of platforms in the station may improve the traffic density, however, the using frequency of the platform may be reduced when the train interval is large.