The invention discloses a seabed soft-soil layer preparation device and method for a large-dimension physical model test, and belongs to the technical field of geotechnical engineering tests. The seabed soft-soil layer preparation device comprises a consolidation and sample preparation system, a monitoring and control system, and a counter-force loading system, wherein the consolidation and samplepreparation system is pressurized from the upper part under a certain water-head condition, and consolidates a soil sample under a double-sided water draining condition, and density of the soil sample is tested; the monitoring and control system can monitor state changes of the soil sample in real time through sensors arranged in a soil sample consolidation region and an upper loading region, sothat various physical and mechanical parameters, such as moisture content, soil pressure, pore water pressure and a pore space ratio, of the soil sample are obtained; and the counter-force loading system can apply stable and uniform consolidation pressure to the soil sample consolidation region, and can reduce and even eliminate influences of unsymmetrical pressure. Three systems cooperatively work, so that real-time feedback is realized, and sample preparation ends while needed sample preparation standards are reached. The device is strong in applicability, is easy in connection, is simple and convenient in operation, is good in sample preparation integrity, is uniform and consistent, and is also suitable for seabed soft soil and ultra-soft soil sample preparation.