The invention discloses an ultrasound-based freight car adaptive load detection system, which comprises a main control processor, an ultrasonic sensor, a temperature sensor, a six-axis acceleration sensor, a car CAN bus, a communication module and a background data center, wherein the ultrasonic sensor, the temperature sensor, the six-axis acceleration sensor and the car CAN bus are connected withthe main control processor respectively; and the main control processor is connected with the background data center through the communication module. The invention also discloses an ultrasound-basedfreight car adaptive load detection method. In no load, ultrasonic signals are transmitted, returned signals are transmitted to the main control processor, and a measurement distance D is calculated;in load, ultrasonic signals are transmitted repeatedly, and a load distance D' is calculated; and according to a distance change amount deltaD, the load weight W is calculated. The system and the method disclosed in the invention has the advantages that mounting is convenient, acquisition is quick, the data are accurate, the cargo weight change and the car tilt condition can be detected and reported in real time, and remote management and supervision are carried out.