Abstract: The stress mechanism of multi ribbed floor slab under vertical load is analyzed. Analysis shows that the results are a bit different among of different software because of different calculation principle. So the stiffness of column cap needs to be calculated. And the main beams and the moment of the first beams connected with the column caps are affected greatly by the column caps; but the other beams are less affected. Because the column caps are equivalent to a rigid component，which can effectively decrease the calculation length of the beams. So the column caps design needs to be well done in multi ribbed floor slab design. The diagram of the bar of column cap is shown. And the bars of main beams need to be adjusted according to the column caps influence. And the results also show that the stiffness of beam is increased because the slab acts as a flange. So the effect of strong column and weak beam can not be easily achieved.