[en] Generally, when β radioactive samples are being measured, the counting rate of β particle shooting out the sample's surface in some thickness may be got. But what people want to know is how many are there β radioactive activity in the sample. In order to solve these problems in theory and experiment, this paper researched the quantity relation of source efficiency ε_s equivalent mass attenuation coefficient μ_m and the sample's thickness X_m. The β radioactive source efficiency of ¹⁴⁷Pm, ⁶⁰Co, ¹³⁷Cs, ⁴⁰K and ⁹⁰Sr-⁹⁰Y are measured. After the self-absorbing in radioactive source is taken into account, the relation between source efficiency and equivalent mass attenuation coefficient is derived on theory. The results of experimental and academic data are very coincident, and this relation formula is approximately suitable for fission-products. (author)