Using some formulea about relations between source parameters and ambient shear stress 70 deduced by Chen Pei-shan, the 7o for 60 earthquakes（ML≥2.5） occurred in South China are calculated, based on the data of Guangdong Digital Telemetry seismic wave with the source parameters ： seismic radiated engergy E, comer periodfo, source peak acceleration ap, source peak velocity vp, source peak displacement dr and seismic moment Mo. The analysis results of the relative errors and correlations for estimated To values from different methods shows that the smallest relative error δT0/T0 is （5 - 15 ） % , when using the source peak acceleration ap, and the correlative coefficient is maximum. For the digital data, T0 could be calculated by ap directly, without seismic moment Mo for which calculation is complicated. So the method of calculating the ambient shear stress with source peak acceleration ap is simple and reliable.