Juveniles Chinese sucker(Myxocryprinus asiaticus)obtains schooling swimming behavior.However,it is unclear when and why they form schooling behavior.We researched the mechanism of schooling formation related to hydraulics.We analyzed swimming behavior of Chinese sucker that ranged from 5 to 39 days post hatching(dph)at 20 ℃ water temperature by video record.Swimming speed,nearest neighbor distance(DNN),separation swimming index(ISS)and the Reynolds number based on total length(RL)were calculated.ISS decreased significantly with age,significantly lower than random distribution value at 19 dph(1.77 cm in total length,TL)and became stable at the lowest value at 32 dph(2.40 cm,TL).DNN changed in a similar way to ISS.The variation of ISS and DNN illustrated the schooling behavior was formed at the 19 dph and became tight schooling at 32 dph.The total length increased with age at function y=1.260 33-0.015 58×x+0.003 15×x²-(4.849 78E-5)×x³.Swimming speed first increased and then decreased and finally became stable at 2.45+0.15 TL/S after 19 dph.RL increased with age at function y=-607.891 61+156.809 12×x-4.212 9×x2+0.037 9×x3,RL exceeded 200 at 5-6 dph,equaled 1 111 at 19 dph.Analyses showed that the RL did not explain the schooling behavior of Chinese sucker,and schooling behavior was more likely related to the swimming behavior,as swimming changed from intermittent sprinting to continuous cruising when schooling was formed.