•A simple, label-free and turn-on fluorescence method was presented.•The NMM/G-quadruplex structure is used as a label-free signal reporter.•The sensitivity of this method was very high and the detection limit was 12.5 nM.•This method was applied to detect melamine in raw milk and milk powder. Melamine has a high nitrogen content thus is often illegally added to milk powder to phonily boost the protein percentage composition. Milk products containing melamine are harmful to babies and children. In the present work, a simple, label-free and turn-on fluorescence method to detect melamine with a high sensitivity and selectivity has been developed. The N-methylmesoporphyrin IX (NMM)/G-quadruplex structure is used as a signal reporter in this method to realize label-free detection of melamine. The detection limit of this method is 12.5 nM, which is much less than the maximum level permitted for infant formulas. The linear range is 0.05–1 μM (R2 = 0.98). The selectivity of this method is also very satisfactory. Furthermore, this method can be successfully applied for detection melamine in raw milk and milk powder.