Abstract: Abstract：Red mudstone fill material (RMF) is highly sensitive to water content. Evolution on mechanical properties of RMF subgrade during long-term relative humidity cycles would be induced by the change of seasons. The long-term relative humidity (RH) cycles (250 days) with various amplitudes were imposed on RMF. After RH cycles, the unconfined compression, bender element tests and cyclic triaxial tests were performed to investigate the degradation on unconfined compression strength and small strain stiffness. In addition, the Scanning Electronic Microscopy (SEM) tests were employed to examine the microstructure evolution of water dripped specimen during wetting and drying. For the high amplitude of RH cycles, the specimens on the dry side (low RH) have shown shrinkage that and higher strength compared to initial state. The swelling and lower strength is observed in specimen on the wet side (high RH). Degradation on the strength, small strain stiffness and resilient modulus have been clearly observed with RH cycles. For the low amplitude, the specimen has swollen on both wet side and dry side, causing a progressive reduction in strength and small strain stiffness. A unified framework is proposed capture the evolution on strength and stiffness with irreversible volumetric strain under different amplitude of RH cycles. SEM micrograph of water-dripped specimen indicates that the degradation on mechanical properties can be mainly ascribed to irreversible slaking of aggregate and crack propagation.