The invention relates to the field of foundation treatment, in particular to a spiral self-drilling steel pile capable of being reinforced through diameter increasing, and a construction method of thespiral self-drilling steel pile. The spiral self-drilling steel pile capable of being reinforced through diameter increasing comprises a spiral pile with a center shaft being provided with spiral blades, and further comprises a lengthening rod composed of one or more lengthening sections; the upper end of the spiral pile is connected with the lower end of the lengthening rod, wherein diameter-increasing guiding discs are arranged on the one or more lengthening sections in the lengthening rod; and the diameter-increasing guiding discs are each in a funnel shape, openings of the diameter-increasing guiding discs face upwards, and the bottoms of the diameter-increasing guiding discs are connected with the rod wall of the lengthening rod in a sealed mode. Through pile side grouting, slurry reaches the diameter-increasing guiding discs to form a reinforced body, the vertical limit bearing capacity, critical buckling load and transverse carrying capacity of the pile are improved, and compared with the mode for improving the bearing capacity through methods of increasing the number of the blades, the diameter of the blades and the pile length, the more economical and applicable effects are achieved so as to enable a foundation to be more stable.