The invention provides an online remeasurement method for hot billet sizing. A reflecting phototube set is mounted on a front roller bed of a flame cutting machine; when the flame cutting machine and a slab are moved to a slab head and the reflecting phototubes are triggered, signals are transmitted to a primary computer; the primary computer does recording and calculates the length of the slab; remeasurement results are compared with shooting sizing results by a shooting sizing server; if the length deviation is smaller than +/-20mm, normality is confirmed; if the length deviation is larger than +/-20mm, an alarm is given; if three slabs are successively overproof, shooting sizing correction is regulated. The method is simple, feasible, accurate, practical and highly stable and has the advantages that system interference resistance and sizing accuracy are improved greatly, sizing pass rate is increased, specified length of each hot billet can be reduced by 10mm, and economic benefit is significant.