The invention provides a main pipe of a water separator, belonging to the technical field of machinery. By adopting the main pipe of the water separator, the problem that the circular center of each water separation hole in the main pipe of a current combination-type water separator is difficult to position is solved. The main pipe comprises a first branch pipe and a second branch pipe which are coaxially arranged and are connected through threads; a plurality of first water separation holes are formed in the first branch pipe axially; a plurality of second water separation holes are formed in the second branch pipe axially; the outer side of the end, which is connected with the second branch pipe through threads, of the first branch pipe is provided with a convex relying part; the outer side of the end, which is connected with the first branch pipe through threads, of the second branch pipe is provided with a convex baffle part; and part of the baffle part extends out of the end surface, which is connected with the first branch pipe, of the second branch pipe along the axial direction of the second branch pipe; and when the relying part relies on the extending part of the baffle part on the second branch pipe, the circular centers of the first water separation holes in the first branch pipe and the circular centers of the second water separation holes in the second branch pipe are on the same straight line. The main pipe has the advantage that the circular centers of the water separation holes can be rapidly positioned on the same straight line.