As an important parameter for the design of abruptly expanding stilling basin, length of hydraulic jump has remarkable effect on the safety, stability and economy of the stilling basin. The variation of length of hydraulic jump in abruptly expanding stilling basin was researched through establishing motion equations of flow particle in hydraulic jump zone. Furthermore, a half-theoretical formula for the hydraulic jump length was put forward and measured data from existed literatures was employed to verify the formula. Result shows that the length of hydraulic jump is the function of four parameters, namely upstream Froude number, upstream mean flow depth, conjugate water depth ratio and abrupt enlargement ratio. With the increase of the four parameters, the length of hydraulic jump increases. The mean error of length of hydraulic jump calculated by the proposed equation is 4.32%. Among 48 working conditions of five patterns, the relative errors of only five conditions exceed 10%, with the maximum value of -12.5%.