The invention discloses a method for treating a subgrade of a highway in a frozen soil region. The method comprises the following steps: a, thawing a frozen soil subgrade in a frost line in advance; b, excavating all thawed soil; c, laying a layer of coarse-grained soil and a reversed filter layer at the bottom of an excavation layer, and laying lime waterproof layers and gravel expansion guide layers on two sides; d, filling activated magnesia carbonized and cured sludge soil layer by layer and performing tamping reinforcement; e, compacting filled embankment filling soil and setting a coarse-grained soil layer containing an XPS insulation board below a pavement structure; f, setting waterproof protecting feet on two sides of an embankment respectively. Subgrade pre-thawing, filling treatment and soft soil carbonization and curing technologies for the frozen soil region are fully combined, the frozen soil subgrade is effectively reinforced, the engineering problems of refreezing and frost heaving of subgrade soil are well solved, non-uniform settling of the subgrade is further reduced, and the problem of secondary damage of the highway in the cold region is reduced greatly. The method has the advantages of being economic, environmentally friendly, safe, efficient, high in controllability, easy to operate and the like.