Thanks for your really helpful and insightful reply.
It might be my mistake to give you the wrong model, I will take a look at my model and get back to you as soon as possible. But very thankful for your help.
1. As you stated "You can see 99% of the mass moves with mode 1, while only a tiny fraction of the mass moves with any of the higher modes." But from the cumulative mass fraction, It seems that even the higher mode, there are almost 99% mass participated into the movement, While from the Ratio Efft.Mass to Total Mass, it seems that as for the mode 1, 86.257%(not 99% as you mentioned) mass will be participated into the move while only a tiny fraction of the mass moves with any of the higher modes.
2. As for the base acceleration, I think you have to use the mode superposition which is a linear solution, Am I correct? If it is the case, then what is the difference between the base acceleration as the input and the acceleration applied to the whole body(As what I did in transient structural with the fixed support and the acceleration) within ANSYS Workbench?