Through corelation analysis and mathematical model computation of the observed data of earth resistivity with different electrode spacing carried out at Daboshe Station, Longyao County of Hebei province, a physical explanation of the annual variation of earth resistivity is proposed. It is highly probable that the annual variation of earth resistivity is caused by the effect of change in ground water level resulting from the close electrode spacing used. To avoid such interference, the electrodes should be spaced at least 500 meters from one another (the overall dimension should be at least 1500 meters) at Daboshe Station.