In order to fulfill the information about long period oscillation in Hualien Port, this study had extended the sampling interval from conventional 1200 seconds to 15000 seconds with 2 Hz sampling rate for the synchronous wave measurements in both Yen-Liao fishery port and Hualien Port. By using the digital filter analysis and FFT, the waves are found, in time domain and frequency domain, to contain the component waves with periods longer than 1000 seconds in ordinary waves, and the wave energies between wave periods of 100 seconds and 500 seconds were strongly increase. The results show that extending sampling interval is helpful for collecting the wave information in a harbor, and the results also lead to a new possibility about the mechanism of harbor resonance in Hualien Port.