Diapause, an important developmental process in insects, is a physiological adaptation for avoiding adverse environmental conditions. Laodelphax striatellus Fallén is an important graminaceous crop pest in East Asia, and there are few reports regarding the nymphal diapause from fields with rice–wheat rotation. In the present study, we determined the fourth-instar nymph as the main diapause stage through investigation under field and laboratory conditions. Developmental duration of the third and fourth instars was longer during the short day-length (10 h light:14 h dark [10L:14D]) at 20°C than during other photoperiods. The third-instar nymph is the most sensitive stage to changes in the photoperiod. The optimal environmental factor for diapause termination was long day-length (16L:8D) at 25 to 28°C. The supercooling point was significantly reduced in diapause nymphs, and activities of trehalase, pyruvate kinase, and sorbitol dehydrogenase were significantly decreased (46.46, 37.90, and 17.64%, respectively). The information obtained in this study may be beneficial to the development of control strategies for this pest.