The invention discloses a combined radiation source strength backward induction method and a combined radiation source strength backward induction system for a linear source and a surface source of a nuclear power plant. The method is characterized in that dosage rates at a plurality of positions are acquired through a detector; a normalized radiation source strength is diffused in a space; an optical distance is calculated by means of a ray tracking method; an equation set coefficient is calculated according to information such as material and accumulation factor, thereby performing backward induction for obtaining source strength; then dosage rate calculation is performed on detector position; through performing linear regression analysis on a measured value and a calculated value, key parameters such as standard deviation, gradient and intercept are calculated, and furthermore a quality factor is calculated for measuring acceptance degree of a calculation result at each time. Furthermore the invention provides a weighted iteration method. An error which is introduced by a detector with relatively high uncertainty is reduced. The steps are repeated by multiple times in an iteration manner until the quality factor reaches a preset fixed value, and anticipated radiation source strength information is obtained.