The invention relates to a nuclear main pump runner forming method based on intermediate line control. A nuclear main pump runner is formed by rotating a disc side runner line and a cover side runner line of a nuclear main pump around the axle wire by one revolution. The method comprises the following steps: S1, determining a runner intermediate line equation; S2, performing discrete treatment on the runner intermediate line; S3, calculating coordinate points of the disc side runner line and the cover side runner line according to a runner intermediate line control point; and S4, forming the nuclear main pump runner. The nuclear main pump runner is formed by controlling the runner intermediate line. Fast programmed design of the nuclear main pump runner can be realized while smoothness and high quality water through characteristics of the runner are guaranteed; a proper runner intermediate line trend can be flexibly selected and runner forming is implemented according to the mechanical characteristics of the nuclear main pump. The method provided by the invention further can provide reference for fast design of high quality water through characteristic of other diagonal flow runners and centrifugal impeller runners.