The concentration field of a double-layer impinging stream mixer was measured by the planar laser-induced fluorescence (PLIF) technique. The multifractal characteristic of the concentration-time series was investigated by using the multifractal detrended fluctuation method (MF-DFA). The multifractal singular spectrum under different nozzle spacing and jet Reynolds number was analyzed to realize the quantitative characterization of flow field characteristics. The results showed that the singularity of the concentration-time series increased with the jet Reynolds number. With the increase of the nozzle spacing, the singularity of the concentration-time series became weakened, and the influence of nozzle spacing on the singularity of concentration-time series was more significant than that of Reynolds number. The stronger the singularity of concentration signal of the double-layer impinging stream mixer, the more intense the motion of fluid particles, and the stronger the mixing effect. The promoting effect of nozzle spacing on mixing effect was greater than jet Reynolds number. According to the variation of multifractal spectrum of concentration time series of double-layer stream mixer, the singular spectrum parameters of concentration time series were obtained. Based on the distribution of the multifractal spectrum parameters (Δα, αmin, α0) along the radial position in the multifractal singular spectrum, the flow field in the double-layer impinging stream mixer was divided into axial impingement zone, vortex zone, and radial impingement zone from the impact center downward. The singularity of concentration signal of double-layer impinging stream mixer was larger than the primary impact region and larger than the vortex region. The effect of secondary impact zone on mixing was stronger than that of primary impact area and stronger than that of vortex area. It can provide a theoretical basis for revealing the flow pattern of the impinging stream system and the complex and disordered flow mechanism of the impinging stream.