In the present study, a new flame retardancy and non-slip finishing system in one step was developed for cotton/polyester (Co/PET) fabrics. Recipes including P-N synergetic FR agent, PVP (PR)-P-DCDA (Fire-off), polysilicic acid and citric acid combinations were developed and applied to cotton and Co/PET fabrics by impregnation method. Flame retardancy (FR) properties and seam slippage resistance of treated fabrics with Fire-off/polysilicic acid systems were investigated. In addition, characterization analysis such as Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimeter (DSC) were performed and tensile properties of treated fabrics were tested in order to detect any changes on mechanical properties of fabrics after treatment. Results demonstrated that Fire-off/polysilicic acid treatment leads good FR properties (LOI >27) and seam slippage resistance (warp: 2.2 mm, weft 1.2 mm) with superior ease of application in one-step process.