The non-equilibrium minority carriers produced by the α-particles while penetrating into the diffused layer of the Au-Si surface barrier detector diffuse into the boundary of barrier layer and are then collected. Hence an output pulse is obtained at the two terminals of the detector. The collecting efficiency in the diffused layer and the relations between the collecting efficiency and the diffusion length are calculated. The experimental results have been compared with the theory, and the diffusion length of minorty carriers is then determined.