Au-Ag nanoparticles (NPs) varying in size (35~91 nm, Au core 19 nm; 66~127 nm, Au core 43 nm) were synthesized via a typical seed growth method and characterized for their morphology and optical properties. The Au-Ag NPs were then applied as surface-enhanced Raman scattering (SERS) substrates for imidan analysis and the influences of particle size and gold-silver ratio for SERS detection were investigated. The results showed that 42 nm Au-Ag NPs with 19 nm Au core and 78 nm Au-Ag NPs with 43 nm Au core had the highest SERS enhancement effect for imidan standard solutions, and could be used to detect the imidan solution at as low as 0.05 mg/L. However, the SERS enhancement effects of Au-Ag NPs for imidan in apple juice were quite different.The minimum detectable concentrations of imidan in apple juice were 5.0 mg/L and 0.5 mg/L, respectively, with the use of 42 nm Au-Ag NPs and 78 nm Au-Ag NPs as SERS substrates. This study indicated that it is possible for rapid SERS detection of imidan in fruit juices by selecting Au-Ag NPs with suitable particle size and the ratio of Au to Ag.