Aiming to realize high-value utilization of biomass solid wastes such as pharmaceutical sludge and fruit branches, and improve the service performance of sulphoaluminate cement (SAC) in sulfate dry-wet cycling environment of coastal agriculture, tidal flats aquaculture, and coastal farmland water conservancy projects, hydrothermal-pyrolysis biochar ( HPB ) was prepared by co-hydrothermal carbonization coupled with pyrolysis. The effects of HPB content (1% , 3% , 5% ) on mechanical properties and hydration characteristics of SAC mortar were investigated, and 150 sulfate dry-wet cycles were conducted. Results showed that the compressive strength was firstly increased and then decreased with the increase of HPB content. The 28-day compressive strength reached 48 MPa at 1% HPB, 9.1% higher than that of the control. HPB retarded early ettringite formation, promoted late hydration, and optimized microstructure. After 150 cycles, the mass loss of 1% HPB mortar was only 1.09% , much lower than 2.35% of the control, and the performance degradation was delayed by about 30 cycles. HPB significantly enhanced sulfate resistance by inhibiting excessive ettringite growth, optimizing pore structure and reducing sulfate ion penetration. The research result can provide a scientific basis for biomass waste recycling and green modification of SAC mortar in coastal agriculture, tidal flats aquaculture, and coastal farmland water conservancy projects.