Accelerated degradation test (ADT) is one of the most common methods to assessing product reliability life. Compared with a constant-stress accelerated degradation test (CSADT), the step-stress accelerated degradation test (SSADT) has more significant advantages in reducing experiment costs and shortening the total time; and compared with a sequential accelerated degradation test, it has better operability and controllability. Therefore, SSADT has a wide application prospect. However, the previous research of SSADT does not consider the impacts caused by the stress conversion, or assumes there are no effect. In this paper, an experiment using LED samples to observe the degradation at the moment of stress conversion was designed and conducted. The results show that the impacts of instantaneous stress conversion should be considered in SSADT degradation trajectory modeling.