In high repetition rate and high di/dt laser discharges, small concentrations of impurities in the laser tube may cause significant degradation to the laser output. More seriously, the presence of impurities in the discharge may cause dirt spikes on the thyratron, and will result damages to its anode. In this paper, the influences of impurities in laser discharge are investigated from the electrical viewpoints. The generation of dirt spikes is explained by the concept of Townsend discharge and electron attachment. Water vapor is found to be the dominant impurity to cause the dirt spike generation. The dirt spike effects on thyratron operation are studied. A new rating factor, termed as the D_b factor, is deduced from the π_b factor. This factor is proved with accuracy by comparisons with the experimental results. Measurement methods to reduce the damages of thyratron are discussed. Under well-characterized operation, the lifetime of thyratron can certainly be extended by the results of this study.