The invention provides an intermediate billet lateral bending detection method. A light projection cylinder formed by a front light projection section and a rear handle is manufactured, five light projection holes are formed in one side of the light projection section at equal intervals, a light-emitting body with a blue light source is arranged in the light projection section, and an electrical storage device is arranged in the handle. When the detection is performed, blue lights projected from the five light projection holes are projected on a high-temperature intermediate billet, and deviation degree of the center line of the intermediate billet is detected by means of transverse distribution of the blue light points on the intermediate billet. The light projection cylinder is moved to enable the blue light points to transversely move on the moving intermediate billet, and longitudinal lateral bending degree of the intermediate billet is measured by means of a motion trail of the blue light points. By means of the intermediate billet lateral bending detection method, the lateral bending degree of the intermediate billet can be effectively detected, diagnosis and judgement can be performed on the lateral bending of the intermediate billet, then preset adjustment is performed on precision rolling, therefore, steel clamping accidents caused by impact to a lateral guide rule during threading are avoided, processing time and loss on defective products are reduced, and smooth production is ensured.