Abstract: Earthquake is a non-line phenomenon, so many seismologists calculate the fractal dimension of seismic distribution. However, the precision of calculation method is uncertain. Because the space-time distribution of earthquakes is characteristics of limitation, dispersion and point sets, we use the digital simulation of theory models of two-dimension double-rule Contor sets to estimate its precision （or error）,and to judge some methods good or not. From the theory models simulated digitally,we obtained some results.① As the samples increasing, the precision of several methods is increasing.② The error of the fixed radius method （RAD） is too big,while ones of the fixed mass method （MAS） and the minimal spanning tree method （MST） is smaller.③ When the numbers of samples are more than 200,the calculation precision of MAS and MST is around 0.05.

Key words: Multi-fractal, Precision, Theory model, Numbers of samples