The invention discloses a nondestructive diagnosis method for rebar corrosion of an iron tower foundation structure in a coastal area, and belongs to the technical field of iron tower maintenance in the coastal area. The method comprises the following steps: selecting at least 5 detection points of a soil layer surrounding a standard sample; determining a mass percentage content of Cl-, humidity and a volume percentage content of the air at the position of at least 5 detection points; performing actual diagnosis on at least 5 detection points to obtain a standard curve; performing mathematicalfitting to obtain a first function formula, a second function formula and a third function formula between a corrosion percent and the mass percentage content of Cl-, the humidity and the volume percentage content of the air; performing merging fitting to obtain a fourth function formula which takes the mass percentage content of Cl-, the humidity and the volume percentage content of the air as an independent variable and takes the corrosion percent as a dependent variable; determining the mass percentage content of Cl-, the humidity and the volume percentage content of the air at a detectionpoint to be detected; and substituting the above into the fourth function formula to obtain the corrosion percent at the detection point to be detected. The diagnosis process is simple, and the result is accurate.