Disclosed are an assembly-type T-shaped mixed coupled wall and a construction method thereof. The assembly-type T-shaped mixed coupled wall comprises a first wall unit and a second wall unit which are connected together perpendicularly and form a T shape. Both the first wall unit and the second wall unit are prefabricated parts, and the first wall unit and the second wall unit are connected through a joint post-pouring belt. According to the assembly-type T-shaped mixed coupled wall, steel beams are used for replacing concrete beams and section steel columns are used for replacing edge constraint reinforced concrete embedded columns, so that the mixed coupled wall has better energy-dissipating capacity and progressive-collapse-resistant capacity than a traditional coupled wall; prefabrication and on-site installation are conducted at the same time, therefore, the problems that the quantity of wet construction on a construction site is large and a lot of construction waste is generated are solved, building operating requirements can be met conveniently, and assembly procedures can be simplified conveniently; after butt welding is conducted on joint short beams, high rigidity is formed between the wall units, therefore, the stability of wall bodies in the construction process is facilitated, the procedure that a shear wall is prefabricated to be used as a temporary support is omitted and the construction efficiency is guaranteed. The construction method can be widely applied to construction of mixed coupled walls in constructional engineering.