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Characters of head of low head pump station and the pump shaft power are analyzed. Influence of each single factor on pump shaft power is expressed as change of specific shaft power (non-dimensional) and the probability density function is determined. Influences of multiple factors on pump shaft power are analyzed. Method of calculating none over-loaded probability of motor by integration by successive reductions is put forward and then relation between power spare coefficient and none over-loaded reliability of electric motor is established. Influences of all factors on pump shaft power being considered completely; power spare coefficients of motor are calculated in three kinds of heads (changing and unchanging), two kinds of dirty-out conditions. Electrical motor power spare coefficients should be chosen as 1.20～1.44, 1.11～1.19, 1.09～1.14 respectively when pump heads are 4, 7, 9.5 m. The results mean much to reasonable choose of electrical motors in large pump stations, increasing reliability of pump units and saving equipment investment.