The invention relates to a prefabricated groove for a deep foundation pit reinforced concrete support beam to be subjected to blasting demolition. The technical scheme is as follows: a prefabricated groove (2) for blasting demolition is excavated in bed rock (3) just below the deep foundation pit reinforced concrete support beam (1) to be subjected to blasting demolition; the longitudinal central line of the prefabricated groove (2) for blasting demolition is parallel to the longitudinal central line of the deep foundation pit reinforced concrete support beam (1) to be subjected to blasting demolition; and the longitudinal central line of the prefabricated groove (2) for blasting demolition and the longitudinal central line of the deep foundation pit reinforced concrete support beam (1) to be subjected to blasting demolition are located on the same vertical plane. The cross section of the prefabricated groove (2) for blasting demolition is in the shape of an inverted trapezoid, wherein the top width a1 of the inverted trapezoid is equal to a+Delta a1, the bottom width a2 of the inverted trapezoid is equal to a+Delta a2, and the height b1 of the inverted trapezoid is equal to Delta b; and the length L1 of the prefabricated groove (2) for blasting demolition is equal to L+Delta L. The prefabricated groove disclosed by the invention has the characteristics of being convenient and fast to construct, good in blasting crushing effect, low in unit explosive consumption and low in blasting vibration intensity.