Abstract: To solve particle crosstalk problem in measuring spectra in a β/γ mixed field, a prototype Phoswich detector with three scintillators was developed. The β and γ were discriminated successfully by the pulse shape discrimination algorithm developed in this work. What’s more, the method to obtain spectra of the two rays was investigated. In the β sources (¹⁴C, ³⁶Cl and ⁹⁰Sr/⁹⁰Y) experiments, the probability that a β particle was determined mistakenly as a γ is not more than 0.1%. In the γ sources (²⁴¹Am, ¹³⁷Cs and ⁶⁰Co) experiments, the probability that a γ particle was determined mistakenly as a β is under 1%. Through experiment of β/γ mixed sources, it is proved that the prototype detector could discriminate the type of β/γ and obtain spectra of the two rays respectively by only one-time measurement.