Abstract: An inversion method is proposed to best fit the observation in frequency domain with synthetic amplitude spectra of full waveform in seismic source mechanism studies. Grid search is performed for finding fault parameters by correlating theoretical spectra with observations. Employing particle swarm optimization algorithm,stable and reliable solutions can be obtained in shorter time. Numerical test shows that,in the case of large location error and worse station coverage,the inversion can result in the solutions close to the given true focal mechanism,and the focal depth near the real focus. The method has been applied to processing local and regional recordings of the 17 May 2010 ML4.0 earthquake in Bohai Sea region. The result is in agreement with that obtained from P wave polarity analysis. We also estimated focal mechanism solutions of 14 MLge;4.0 earthquakes in Shandong peninsula and its adjacent sea area from 2003 to 2010.