In order to guarantee the regulation effect of the downstream channel of the junction and make it reach the standard of planned channel grade，we take Longxikou in the Minjiang River to Yibin 81km channel regulation project as an example，analyze the possible design discharge of upstream junction operation，and study the channel scale under different design discharge by plane two-dimensional flow model. The results show that in the future，the average discharge in dry season （November to next year April） can be increased to 1 757 m3s through the junction operation of the main and tributary reservoirs in the upper reaches of the Minjiang River. When the upstream daily average discharge is more than 900 m3s，the downstream discharge of Longxikou can be more than 900 m3s by reverse regulation. When the discharge is 900 m3s，the navigation depth of the lower reach of the Minjiang River can satisfy the requirements of 2.4 m navigation depth of class III channel. For these reasons，the design minimum navigable discharge of Longxikou in the Minjiang River to Yibin 81km channel should be 900 m3/s.