Based on random Poisson distributional model and Gutenberg-Richter law, we made 8 groups of simulated seismic catalogues with the sample size of 1 000 to 15 000 within 20 years so that we could discuss the effect on the H value of frequency sequence of seismicity caused by the calculational conditions of sample size and τ_max of maximal fitting window length. The result shows that the whole precision and stability of H value improve with the sample size increasing. When τ_max is equal to 300, it is specially obvious. Case calculations show that it is available to pick up well the process of anomaly change of H value before strong earthquake with selecting reasonable calculational conditions.