The invention discloses a drilling method for a sidetrack horizontal well isodiametric with an original wellbore.The drilling method includes the steps of subjecting an oil well to pre-drilling treatment; starting to perform sidetrack drilling at a sidetrack drilling point position below a surface casing shoe of the oil well; drilling for a first preset distance according to a hole-angle-increasing drilling technology; drilling for a second preset distance according to an angle-holding drilling technology; drilling for a third preset distance according to a hole-angle-decreasing drilling technology to enable a hole drift angle to be decreased to a first preset angle prior to continuing to drill to reach a first preset depth; drilling to enable the hole drift angle to be increased to a second preset angle according to the hole-angle-increasing drilling technology, and continuing to drill to reach a second preset depth according to the angle-holding drilling technology; drilling to reach a horizontal target beginning point according to the hole-angle-increasing drilling technology; drilling to reach the end point all the way from the target beginning point according to the angle-holding drilling technology.The drilling method for the sidetrack horizontal well isodiametric with the original wellbore has the advantages that drilling difficulty is reduced effectively through a smooth drilling trajectory during practical drilling while an old oil well trajectory and a new oil well trajectory are taken into consideration.