Abstract: In order to improve the scientific and accuracy of calculation for risk of loss of a human life, and to give constructive guidance opinion to safety arrangement of manual grounding devices,by analysising on calculation method of touch and step voltages when leakage current by manual grounding devices,determined the quantitative calculation method of the probability of injury to living beings caused by electric shock due to touch and step voltages.In calculation,by the comparative analysis between the calculated value of touch and step voltages and threshold value of human discomfort tolerance to touch and step voltages to determination the minimum lightning current amplitude in order to reach the threshold value,and by the analysis on characteristic of thunderbolt activity at the location of the project to determination the lightning current amplitude exceeding to corresponding to threshold value,that is the proportion of the lightning current producing risk,and holds that the proportion is probability of injury to living beings caused by electric shock due to touch and step voltages. Calculation of ouch and step voltages to artificial ground substance with vertical grounding rod as exemple,and the influence of the different between impulse current and power current is considered.