The invention discloses a method for detecting depths and lengths of soil body cracks by relative variation of resistance. The method comprises the following steps of: defining a logarithmic resistance ratio, a crack depth ratio and a horizontal crack length ratio; testing indoors, namely arranging vertical electrodes in a pair at two sides of a crack, charging the vertical electrodes with alternating current, measuring the voltage between the two electrodes, calculating the resistance, and converting the calculated resistance into a logarithmic resistance ratio, thereby obtaining relationship curves of the logarithmic resistance ratio and the crack depth ratio as well as between the logarithmic resistance ratio and the horizontal crack length ratio; determining in field, namely measuring voltage similar to that of the indoor determination, calculating resistance, and converting the calculated resistance into a logarithmic resistance ratio; calculating crack depth according to properties of the relationship curve of the logarithmic resistance ratio and the crack depth ratio which are determined in advance; and acquiring crack development length according to properties of the relationship curve of the logarithmic resistance ratio and the horizontal crack length ratio which are determined in advance. According to the invention, the resistivity of soil bodies is prevented from being calculated directly, and the situations of development size and the like of soil body cracks can be obtained through the relative variation of resistance.