A compact shifted excitation Raman difference spectroscopy (SERDS) multi-wavelength light source based on tunable external cavity semiconductor laser (ECDL) is designed. A wavelength tuning range of 15 nm can be achieved, with the laser spectral line-width less than 0.2 nm and output power up to 80 mW. We also developed a deconvolution algorithm with multiple constraints to process the difference spectrum of SERDS, which can suppress the noises efficiently. This SERDS system is used to test several illegal food additives (sudan I, rhodamine B, melamine, hexamine). The results show that this method can effectively remove the background fluorescent and significantly increase the signal-to-noise ratio of Raman detection.