Torsion strain spectrum refers to the variation of the torsion strain of a specimen with external field, such as temperature, electric and magnetic fields, which facilitates the research of its dynamic behavior. We demonstrate that a torsion strain spectrum can be achieved in a high accuracy by making use of the conventional inverted torsion pendulum. By measuring its torsion strain spectrum of ferroelectric relaxor PMN-32%PT, we verify that its tetragonal-cubic transformation belongs to a burst-type martensite transformation.