Visible and near infrared (Vis-NIR) spectroscopy was used to accomplish a rapid and noninvasive quantification of the two common adulterants, soybean oil and rapeseed oil, in fish oil. Different contents of soybean oil were added into fish oil of three brands and different contents of rapeseed oil were added into fish oil of another three brands, the Vis-NIR spectra of adulterated samples were collected, pretreated by five spectral preprocessing algorithms (smoothing, standard normal variate (SNV), multiplicative scatter correction (MSC), 1st-derivative, and 2nd- derivative), and used to establish partial least square regression (PLSR) models. The correlation coefficients for prediction (Rp) of 0.938 6 and 0.959 3 were obtained for the adulterant detection of soybean oil and rapeseed oil respectively, and their optimal models were full range spectral PLSR model and MSC-PLSR model. Successive projections algorithm (SPA) was then used to analyze the full range spectra of fish oil samples adulterated with soybean oil and rapeseed oil respectively, and 11 and 15 spectral characteristic wavelength variables were obtained. The Rp of 0.941 2 and 0.932 6 were obtained based on SPA-PLSR models for the adulterant detection of soybean oil and rapeseed oil, respectively. The overall results indicate that Vis-NIR spectroscopy is a feasible way to determine the adulterants of soybean oil and rapeseed oil in fish oil rapidly and non-destructively.