The invention discloses a detection method for a sinking position of a flexible mattress. The detection method is characterized by comprising the steps of adopting the GPS (Global Position System) positioning systems of a base-station and three mobile radio stations; installing the three mobile radio stations on a measuring ship and moving above a detection point; installing an ultrasonic transmission sensor at the edge of the mattress where the detection point is set, and installing the ultrasonic receiving sensors all around the three mobile radio stations; receiving and measuring the propagation time that ultrasonic waves are arrived by the three ultrasonic receiving sensors, and calculating distances; meanwhile, determining a three-dimensional coordinate of the three ultrasonic receiving sensors by the information of the mobile radio stations, and obtaining the space position of the detection point through the interpretation of the space coordinate. According to the detection method provided by the invention, the sinking position of the flexible mattress can be quickly and accurately obtained to provide a basis for the subsequent sinking, and the sinking effect of the flexible mattress can be ensured; meanwhile, the detection method has the advantages of being simple to operate, low in cost, good in feasibility and high in precision.