Abstract: Measuring the source parameters of small earthquakes is a basic work to study the source and regional tectonic stress states. This study uses the recently developed Generalized Polarity and Amplitude Technique (GPAT) for solving the focal mechanism of small earthquakes above M_L2.0 of Tangshan earthquake swarm on August 20, 2016. The results show that: ① The focal mechanism of the earthquake swarm is dominated by normal-fault types, accounting for 74.6% of the total. ② For earthquakes smaller than M_L3.6 in the earthquake swarm, the moment magnitude M_W obtained by GPAT is larger than the M_L magnitude commonly used in the observation report of the network, the smaller the magnitude of the event, the more obvious the gap, the maximum magnitude deviation is 1.0. ③ The GPAT depth reflects the true depth of the source. In short, the GPAT method works well in the inversion of the focal mechanism of the Tangshan earthquake swarm.