Based on terahertz time-domain spectroscopy and prism attenuated total reflection technique, we design a non-invasive blood glucose detection system with terahertz wave reflectance spectroscopy. With the help of Debye model, it is found that the change of water content in the skin can affect the relaxation of the skin in terahertz band. The relationship between the complex refractive index of the skin and the water content of the skin is obtained. The method of calculating the real part and imaginary part of skin refractive index is given by means of Fresnel formula. The real and imaginary parts of the complex refractive index of human thumb skin measured by terahertz spectroscopy are compared with the actual human blood glucose concentration measured by oral glucose tolerance test. The relationship between the terahertz reflection spectrum and complex refractive index of human thumb skin are positively correlated with human blood glucose concentration. It provides a new method for the application of terahertz in noninvasive blood glucose concentration detection.