The biological content of crops is directly related to the optical properties of the crops, which are affected by the bidirectional reflectance distribution function (BRDF) of plants. The BRDF of plant leaves reflects the energy reflection ability of leaves in different directions and directly affects the spectral detection results of plant leaves, also one of the influencing factors of macroscopic optical characteristics of vegetation canopy. Studying the BRDF optical properties of plant leaves can effectively improve the stability and reliability of the plant NDT spectral model, thus improving the accuracy and reliability of using the crop spectral model to invert the physicochemical properties. In this paper, we first introduced the BRDF rapid acquisition method for plant leaves and the independently developed directional spectral detection instrument which can adjust the four dimensions of incident light azimuth and zenith angle, receiver probe azimuth and zenith angle to receive the reflectance data under multi-incidence and multi-reflectance angle. The leaf veins of monocotyledonous plants are longitudinally distributed, thus showing more significant anisotropy. Maize and wheat are two typical monocotyledonous crops. Then the self-developed instruments were used to obtain the reflectance spectra of corn and wheat under different wavebands, and their reflection distribution was analyzed and summarized. The spectral data and whiteboard correction data were calculated by using the BRDF calculation method described in this paper. In combination with the image mapping of spectral reflectance data from MATLAB program, the correlation of the reflectance results with two typical physicochemical parameters of leaves, chlorophyll content and water content was analyzed. After analysis, the method of using ANIX coefficient to quantitatively analyze the anisotropy of blade was discussed. The data of wheat in the visible light band and corn in the near infrared band were selected for final analysis. The results show that the fr distributions of wheat and corn in each band are symmetrical about the tiny space on both sides of the incident zenith angle, and the fr values under different incidence zenith angles are basically the same at the same band. Under the same incident zenith angle, wheat has the largest fr value at 800 nm, and the smallest fr at 680 nm. This is due to the strong absorption of chlorophyll near the 680 nm wavelength, and the strong reflectance near 800 nm. The increase of chlorophyll concentration will lead to the increase of fr value under the same band. At 1450 nm in the strong absorption band of water, the fr value of corn increases with the increase of water content. The analysis shows that the BRDF characteristics of crops can effectively reflect the changes of the main biological content of the leaves, and the calculated anisotropy index also shows a consistent change law, which provides a theoretical and practical basis for establishing a stable and reliable model for the quantitative analysis of crop spectra.