The balance principle of the straingauge arrays observation is analyzed in detail according to the first invariant of the stress. A partly damaged straingauge arrays' strain conversion is discussed. The stress computation through the strain gauge arrays under different condition is confirmed. A method for determining the validity of the straingauge arrays is studied and summarized. A collection procedure containing the straingauge arrays' validity judgment, strain conversion, and stress computation is established based on the causal relation model.