The invention relates to an instrument and method for testing the mechanical property of materials under tension-torsion composite-force-thermal coupling working conditions, and belongs to the field of precision instruments and equipment. The instrument includes a supporting module, a tension loading module, a torsion loading module, a high temperature loading module and an in-situ monitoring module; and a torsion motor of the torsion loading module is fixed to one side of a first helical gear reducer and the other side of the first helical gear reducer is fixed to an upper base of the supporting module through fixing bolts, so that a sample to be tested is subjected to bidirectional and simultaneous tension loading. The tension-torsion combined load loading and test can be performed on the sample to be tested under high temperature environment, the mechanical behavior and performance evolution law of the materials under the action of high temperature and tension-torsion complex mechanical loads are dynamically tested, and the characteristics of stable whole machine structure, rich compatible modules, high test accuracy, high complexity of load loading environment are achieved. Important foundation and support are provided for material development and preparation, optimization design of mechanical equipment, life prediction and reliability assessment.