Abstract: Using a model of circular shear dislocation with the propagation medium non-uniform crack and considering the unit-impulse response of the propagation medium-seismograph system,the amplitudes and half period of the first arrival of P waves radiated in various directions are calculated in this paper. We have as well given a few sets of theoretical curves by which source parameters are determined. Bused on the Q value obtained from the first quarter period of the initial P arrival and the fault-plane solu tion fitted by the focal sphere,a method of simultaneous determination of source parameters and mechanism solutions of small earthquakes is presented. In this method,the source parameters as well as the focal mechanism for individual small earthquakes are obtained by including focal mechanism as part of the parameters to be determined,and by taking its effect on the other parameters into account. Analysis and tests on the data of small earthquakes collected from four seismic stations in the Xinfengjiang reservoir region show that the mean quality factor of the medium is about 450,the seismic moment being 1019-1020 dyn cm and the mean stress drop being 35 bar or so; besides,the faulf-plane solutions represent the principal characteristics of tectonic stress field in this region.