The invention discloses a method for improving sectioning base ring opening precision. According to different points of the method from a conventional technology, for perpendicularity after sectioningbase ring joint close face machining, a steel wire is hoisted to check various parts of the joint close face, a numerical control program is utilized for correcting joint close face perpendicularitydeviation, and precision of the joint close face is guaranteed; 1/2 of a base ring is machined on a boring machine, boring machine stroke precision is utilized, a digital level gage is placed on a boring box, fixed guide vane opening measuring in the whole 1/2 base ring stroke is achieved, and precision of base ring 1/2 datum lineation is guaranteed; a 360-degree rotation center column is utilizedin the integral base ring for measuring and datum lineation and checking, datum uniformity of the integral base ring is guaranteed, and the datum lineation and checking precision is improved; sectioning base ring datum plane machining and combining are conducted, the sectioning base ring is assembled and combined with the datum plane as the reference, and the assembling and combining precision isimproved. The method has the advantages that the base ring opening size control technology is simple, the machining quality is controllable, and precision is high.