Silane coupling agent has been a highly efficient interface modifier, which can form a binding layer of organic matrix-silane coupling agent-inorganic matrix at the interface to enhance the compatibility between organic aggregate and cement matrix. Titanate coupling agent has a good effect in connecting inorganic fillers and polymer matrix. Mild pyrolysis can enhance the hydrophobicity of biomass materials, kill some bacteria, preventing the fungal degradation of biomass materials. Walnut shell has high content of organic matter and poor compatibility with inorganic cement matrix, so it’s difficult to form effective chemical connection in the interface area, which leads to poor mechanical properties of mortar. Moreover, walnut shell is rich in strong polar hydroxyl groups, has a strong ability to absorb water and moisture, and the mechanical properties of saturated walnut shell are reduced, and it is easier to be biodegraded. When used as aggregate, the bearing capacity is greatly reduced. In this paper, walnut shell was modified by three methods, the effect of modification on the structure and properties of walnut shell was investigated, and the properties of mortar with modified walnut shell as aggregate were further studied.