Dear Xiangheng.Feng,
The WAMIT output data is nondimensional. See the following forum post for information on how to dimensionalize the WAMIT output: WAMIT - #4 by Jason.Jonkman. That forum topic links to the WAMIT documentation that clarifies the formatting of the WAMIT output files. The four columns in the *.3 file are the amplitude and phase and real and imaginary components of the nondimensional wave-excitation loads.
Best regards,