The mass absorption coefficient is the basic parameter to determine sediment concentration by gamma ray count ratio in the theoretical computing formula of using gamma ray to measure the runoff sediment concentration. In this paper, a single point method and a multi point method are studied to determine the mass absorption coefficient. As for the multi point method, the regression method to determine the mass absorption coefficient is presented and the regression formula is derived to compute the mass absorbing coefficient using a large number of (multiple points) experimental data. The mass absorption coefficient that makes use of a great deal of measured experimental data to determine the mass absorption coefficient by the regression formula has very high precision and can get rid of the effects of the stochastic errors. However, the single point method can determine the parameter quickly and has specific physical concept because it estimates the mass absorption coefficient by several special points. In addition, the influence of the change of the mass absorption coefficient on the measurement precision of the sediment concentration is analyzed theoretically and it shows that there is direct proportional relationship between the relative change of the mass absorption coefficient and that of the sediment concentration, and the proportion coefficient is 1. Therefore the validity, the advantage, disadvantage and application method of these two methods are determined.