In order to investigate effect of the gain coefficient and core-shell ratio on surface plasmon resonance of nanoparticles, the Au nanostructure with gain medium in the core was numerically simulated. The results show that as gain coefficient of material increases to reach the threshold, the scattering cross section is about 2×105 times of the value of the case without gain. Meanwhile, the absorption cross section turns to negative with the extinction cross section close to zero simultaneously. The electric intensity enhances about 185 times of the case without gain, and the resonant bandwidth decreases from 60 nm to 1 nm. With the core-shell ratio enhanced, both of the extinction peak and the absorption peak are red-shifted. Therefore, the related results provide theoretical reference and technique support on the application of gain medium in surface plasmon resonance.