S3Fit is a python code for the analysis of observational data of galaxies, which can fit spectrum and multi-band photometric Spectral Energy Distribution (SED) simultaneously. It is written to improve the moderate constraints on properties of continuum models in a pure spectral fitting due to the limited wavelength coverage. S3Fit support multiple models with multiple components, and can handle complex systems with a mixed contribution of Active Galactic Nucleus (AGN) and its host galaxy in both of continua and emission lines (e.g., narrow lines and broad outflow lines). The fitting strategy is optimized to enable an efficient solution of the best-fit results for several tens of parameters and model components. S3Fit is also extensible for adding new functions and components by users (e.g., new band filters, new star formation history functions, new emission lines, and also new types of models).