The invention discloses an adjustable steel back arris for formwork reinforcing of a fabricated L-shaped cast-in-situ embedded column and an implementing method of the adjustable steel back arris, andbelongs to the technical field of supporting and reinforcing of building formworks. The adjustable steel back arris comprises an internal corner back arris assembly and an external corner back arrisassembly, the internal corner back arris assembly comprises a first adjusting back arris component, a second adjusting back arris component and a first connecting component, the first adjusting back arris component and the second adjusting back arris component are detachably connected through the first connecting component, the external corner back arris assembly comprises a third adjusting back arris component, a fourth adjusting back arris component and a second connecting component, the third adjusting back arris component and the fourth adjusting back arris component are detachably connected through the second connecting component, and the first adjusting back arris component, the second adjusting back arris component, the third adjusting back arris component and the fourth adjusting back arris component are all adjusted in the vertical direction. Manufacturing is easy, convenient and economical, operation is convenient, adjustability is realized in the vertical direction, reliability is high, the construction period is shortened, and meanwhile the quality is guaranteed.