The invention relates to a novel method for milling a square steel billet to obtain a saddle-type guide vane by adopting a four-axis milling machine. The novel method is characterized in that a square steel billet saddle-type guide vane double end milling process boss tool and a workpiece positioning and clamping jaw tool are designed, wherein the square steel billet saddle-type guide vane double end milling process boss tool comprises a base plate, a middle wall plate, stopping and positioning pins, a pressing plate and a side pressing plate; and the workpiece positioning and clamping jaw tool is characterized by comprising a fixture body, a clamping unit and a positioning unit. The novel method for milling the square steel billet to obtain the saddle-type guide vane by adopting the four-axis milling machine has the advantages as follows: the design is reasonable and ingenious, both big and small process heads are utilized, so that the square steel billet can be accurately positioned and mounted on the numerical control four-axis milling machine, the processing can be carried out constantly until the finished product is obtained by one-time clamping, not only is the processing efficiency improved by several tens of times, but also the processing precision is much higher that of the previous processing methods.